if you've been training well for a couple of months and not eating like an absolute nutter, you may not have put on 5lbs of anything. Some of it may be down to day-to-day weight variation. I agree with DF3 that you are unlikely to have put on 5lbs in muscle no matter what's happened. But maybe you've put on 2-3 pounds in a combination of muscle, fat, and water. Exercising the muscles not only builds muscle but also can increase their glycogen-carrying capacity, which in turn requires more water for storage. (I'm getting slightly out of my depth here to say whether this can and does actually occur over such a short period of time, but it's an indication of how body weight composition is less than straightforward!)
Bottom line: keep training consistently, fuel well but don't overcompensate for marathon training fuel demands by eating too much, or thinking you can get away with eating more bad things. If anything, the increase in exercise makes it more important that the food you eat is good quality and nutrient rich. But do keep using clothing sized as a guide to whether you're putting on any lard.
Edited: 07/03/2013 at 09:26