Loads of sex and being unclean before and after (like not going pee or cleaning yourself up after sex) can lead to a urine infection
, which if not taken care of can lead to a kidney infection
. You probably had a urine infection. When it gets to your kidneys you should be running a fever and in so much pain you are physically disabled in pain. You can get some back pain from a bladder infection as well, but if it were in the kidneys you would be in extreme pain.