how long do you expect the strain to take to finish? if it's an 8 week strain i wouldn't worry about it with just a couple weeks left, but if it's a 10+ week strain you'll want to take care of that now. if the yellowing is rapid you might be leafless in a couple weeks. more than likely you need a bit more nitrogen but without pics it's impossible to say for sure.
a lot of people will feed with the grow formula for the first couple weeks of bloom just to avoid the premature yellowing, or even do half grow/half bloom during that period. some bloom formulas have way too little N to keep leaves green for a long bloom cycle.
just don't freak out and start dumping all sorts of stuff in there. just try a little extra N with the next feeding. if you can get a pic up that would be good, too. i'd hate to be advising you on an N def if something else is going on.