swimming & knee pain?

I used to swim lots in university - havent done much in a while. In the last month i've taken it up again (3) times per week. Since being back in the pool, my knee has been acting up, slight pain, can't sit on it (leg underneath me).

I was told that b/c I swim first thing in the morning the pain is being caused by a lack blood flow. B/c it is so early in the a.m. (530), i was told that the blood hasnt had a chance to circulate as much and the joints haven't woken up.

Your thoughts? could this be the reason for the knee pain?