Glowlight neon tetra - swimming jerky & lump on stomach

[LindsayLePrevost]

Hi, my glowlight Tetra was lurking around at the bottom of the tank hiding, shaking a bit but seemed like it was just eating food off the bottom of the tank. A while later it was doing the same thing so I thought it can't be still eating, it was almost like it was vibrating/jerking (with its head pointing towards the ground). It then came up to swim but it is still jerking as it swims and it's head is pointing slightly downwards. It also has a lump on its belly which I don't think I've seen before. Tank is fine and I haven't had any new fish in over a year, it's been the same 3 fish for a good 6 months. Would anyone know what is wrong?