1 leave starting to turn slightly yellow during flowering? probally not a big deal.
How big is your plant because if its a big plant and its one of the bottom leaves don't worry at all. If its a small plant and its one of the top leaves then there could be a problem.
Also how long has your plant been in the pot it is growing in and when was the last time you fertalized it. Could possibly be nitrogen difficency if you don't fertilize it and it's been in the same soil for over 2 months.