I just started riding after a winter hiatus, on my fourth ride I did 70mi. of rolling hills and after 55mi. I began to experience a tightening of my lower back muscles. I toughed it out and completed the ride, but as soon as I got off the bike I felt lower back spasms.

I researched on back pain and cycling and found that there sometimes is a relation between riding position and back pain, also saw that strengthening your abdominal core will reduce load on lower back.
Those of you who have experienced lower back pain, what were the main causes? How did you alleviate it? Should I feel discomfort and pain after 70mi? Due to road cyclings wind resistant positioning is some back pain expected?

I have to say that I pushed myself very hard on this ride (HTFU) or whatever. I almost vomited twice, but kept pushing. This is my 4th ride within the first week back on the bike, is it possible that I over exerted myself to early?