There are two possibilities that come to mind.  The first one is the one you mentioned.  You may well be gaining muscle while you're losing fat and therefore not seeing the number on the scale go down.  Pay attention to how your clothes fit, if they are getting looser then you are losing fat and getting stronger while you're at it.

The other possibility, and this happened to me when I first started running, is that your appetite has risen to match your level of exercise.  It's one of the sneaky ways your body adapts to increased exercise and it can really sabotage your efforts.  My appetite went nuts when I started running and I actually put on a couple of pounds before I realized what the problem was!  So if your clothes aren't feeling looser start keeping track of what you're eating, that my be where the problem is.