I get cysts all the time and they're not nice.
I'd go back to your GP in case it has become infected and become and abcess.
As Lazygun said it may need antibiotics (or steroid medication or an injection) or have it removed.
Good hygiene is very important, get an antibacterial shower gel or soap such as sanex or carex and make sure your back is washed properly.
A warm compress can help, if it's uncomfortable how about getting something like a ring pillow so you can lean back without pressing on the cyst itself.
Don't try to press on it or stick anything into it or try to drain it.
Hope you feel better soon! x