What's wrong with my handicapped fish?

My fish is little black molly (or tetra, perhaps). She has had many lovers and many babies, and has been in great health her whole life until just recently. Now when she swims, it’s as if a little balloon were tied to her tail – it seems to slowly and rise against her will, and she fights to stay horizontal. Her swimming is obviously very awkward and she struggles to get food. Nonetheless, she’s been eating well and sleeps just fine thanks to a support she has figured out how to use to block and therefore counteract the rising tendency her tail has. What’s causing this, and how can I fix it?