i have lower back problems WHY??

i used to do squats, deadlifts and stiffleg deads for along time now i can not because everytime i do my lower back where the vertabres meet the tailbone hurts like a mofo so bad i can hardly move recently i did only two sets of stifflegs and i had no pain or problems and the next day i could feel the pain setting in and when i reached to put my shoes on there was a pop in my lower back and i fell to my knees in severe pain and laid on the floor for some time before i could mamnge to get myself on my feet?? that was scary the next mourning after that i could not get out of bed i hurt so bad i could not move my back, so today i feel better but i have to be carful or i might lose it agin and take it out. so my question is why do i not feel any pain when i do the movements but it sets in the next day??

REST REST REST until it doesnt hurt and then work your way up in the weights. You cant just jump right into it after tqking an extended amount of time off. Get it checked out by a doctor if you can afford it and if not, like I said, rest until you feel no pain.

after you have recovered I would say start out with exercises not involving the back directly for a couple weeks then try some good mornings, LIGHT WEIGHT. then work you r way up to deads and the weight you use with them. Also try to strengthen your abs as well for they compinsate for your back basically. hamstring curls have even helped me personally and I had a HORRIBLE back.
good luck and be patient.

And to answer why, a doctor would need to see you but it could be a few things. compressed disc, pulled muscle etc.