What you're seeing could be due to an ammonia issue, when the aquarium has ammonia in it you will see fish frantically swimming back-and-forth or up and down trying to find a way to escape. My best guess would be a combination of three things, you did not cycle the 'aquarium' prior to adding the fish, you are not doing water changes, and your 'aquarium' is far too small for even one guppy.
I would advise you to take your fish back to the pet store, buy at least a 10 gallon, cycle it, and try again. Guppies of the same sex require at least a 10 gallon, mixed sex guppies should have a 20 gallon aquarium, filtered and heated.
I recommend doing a fishless cycle. Now, before I even start telling you what this is, I'm going to tell you about some great sources of beneficial bacteria that will help your aquarium cycle faster and just make things much easier. The first one would be aquatic plants, aquatic plants are kept in established aquariums in the aquarium store, when you buy these plants and put them in your aquarium they will transfer it to your tank. The second, would be filter sponges, if you have any friends that have some used filter sponges then that will also be quite helpful with transferring bacteria. It is vital that you use aquarium water, because nothing kills bacteria faster than chlorine. After this is done the water will look brown, maybe even black, but trust me, this is essential for aquariums. After you get the sponges, it must goes straight into your aquarium. Before you dump in the sludge and everything, your water must be dechlorinated. If you do both the plants and the filter sludge your aquarium will be cycled in no time.
Alright, so how to cycle: First you need a source of ammonia some people use a bunch of fish food, but I recommend pure/clear ammonia. The ammonia needs to be free of perfumes, colorants and surfactants. Once you have your ammonia you will need to add it to the aquarium, the goal is to get the ammonia level to reach 5ppm. I would recommend starting with 1/2-1/4 teaspoon, letting it mix through the aquarium and filter, wait about 5 minutes then test the water, do this until your ammonia reaches 5ppm. Make sure to record the amount of ammonia it took to get it up. Add that same amount daily until your nitrite is visible, then you're going to want to half the dose and keep adding it until the nitrite has reached the same level as your ammonia. Once that is done leave your tank alone and let the beneficial bacteria build up and combat the pollutants. Keep testing your water everyday until your levels read: Ammonia: 0ppm. Nitrite: 0ppm. Nitrate Under 10ppm.
Do not add any water conditioner of any kind in your water while it is cycling, it can screw everything up. Also, for a test kit, do not use strips, those also can give false results. I would advise you to get a liquid test kit, either from API or Nutrafin.
When your cycle is done you can then add fish!
Guppies are omnivores, meaning they eat both vegetation and meaty foods. Tropical flakes and/or pellets only make up half of the diet, I recommend flakes over pellets, they are easier to eat in general. The brand I recommend is Omega One Tropical Flakes. As well as flakes you will need to feed vegetables, there are many different vegetables that are good for your fishes health. I recommend any of the following: zucchini, spinach, collard greens, mustard greens, carrots, butternut squash, acorn squash and peas. You will need to blanch the greens first, and you may need to slightly boil the vegetables to get them to sink, or you can tie them to rocks or other heavy objects. I will normally cut up and freeze vegetables for my fish and thaw them out in hot water. I will feed my fish vegetables every 2-5 days. Be very careful not to overfeed, do not keep vegetables in the tank for over 24 hours. For the meat portion of the diet I recommend frozen foods, which can include any of the following: bloodworms, mosquito larvae, daphnia, tubifex worms, black worms, brine shrimp, plankton and mysis shrimp. I feed frozen foods in the morning and flakes at night. You can, however feed flakes one day and feed frozen foods every second day. I do not recommend using freeze-dried foods since they have very little nutrients and are just a waste of money.
You will need to preform 25% weekly water changes. You will need a syphon hose w/ gravel vacuum and a 5 gallon bucket or a python. If you have gravel you need to use the gravel vacuum, if you have sand you will only need the syphon hose to remove the waste. As well as weekly water changes you will need to test the aquarium water for ammonia, nitrite and nitrate every week before a water change, using a liquid drop test kit.