How much should I swim ?

Hi i know that sounds like a dumb question, but I have concerns. i have been reading some of the posts on this site and people are talking of swimming 5 or 6 days a week, I would love to do that. Currently I swim 3 times a week and put in 3,500 yards a session . I would like to increase this but for the last couple of years every time it get going I end up with tendinitis in my elbows and or shoulders. The doctors always say I'm over doing it and to back off. it seems I should be able to up my yardage or amount of swim days but I'm always afraid of injury, I am starting to get a little faster now and can feel my elbows threatening me. Could this be a stroke problem ? Or maybe I'm susceptible to tendinitis ? Any ideas how to get around this and spend more time in the pool would be greatly appreciated. Thank You, Tom