When i got over a large bump or railroad tracks i heard a loud thump coming from the back. It sounds like the read end is getting smashed into my floor pan if nothing else. I just wanted to know if this is normal or if there is something to look out for. I recently put lowering springs, a new sway bar, and new struts in the rear if that makes any difference... i don't remember the sound being there before putting those in but then again i only drove the car for two weeks before replace the aforementioned items.

Lots of people asking questions they know the answer to. SPRINGS.
Don't you have shocks on the rear? Might have put the struts on the front, & did nothing to the rear (shocks).

Anyhow. Your lowered car has less travel now. The rear is hitting the bump stops. Drive slower over bumps, or put the right springs back on. Its not a truck man lol. You should record the sound, & post it on you tube. No one knows your sound from not hearing it. im pretty sure though its too low or you need better lowering springs. Did you ebay the springs?

There Vogtland lowering springs. Vogtland was stupid and sent me the wrong front springs however so my car is two inches higher in the front then the rear. Just making sure what i was hearing was the rear end hitting the ground and not some other unforseen problem (it sounds exactly like the rear end hitting the ground). Thanks for your help.

No problem. Sounds like a bad deal. Good luck.