It is hard to tell from the video is there is something wrong with her or not..
Have you been changing the water?
Did you get the testing kit yet?
Without knowing for sure what the parametres of your water are, it is difficult to advise you. If she IS gasping at the air, that may be a sign of bad levels in your water (too much nitrates, nitrites, ammonia)..
It looked to me like she was playing with the water current from the filter. She may also be restless as a 5 gallon really is too small for mollies.. and there is nothing in the tank for them to interact with. Most fish like to have safe place to retreat, even just a plant or pot or something to hang out near. An empty tank can be stressful for them.
I hope she ends up okay..