Lower Back Pain

The past two Saturdays I have gone swimming for about 30 mins. each time. The first time on Sunday I got up with a sore lower back which woresened as the day progressed. By Monday morning I could hardley walk. I had problems all week. By Friday thing had improved greatly so Saturday I went swinming again. I have had no pain while swimming, but aft getting oout of the water and wihile dressing on the second Saturday I noticed the pain returning. On the secon Sunday I could not get out of bed. I had to roll out of the bed and crawl till I could get up. It took several mins. Here it is Wednesday and I am still have pain even though it is better. I am assuming that I am hyperextending the lower back? Any suggestion as to treatment and what I can do to strengthen so that it won’t occure.