hi there,
my lo has been doing that since she was born..... if I have to wake her up from a nap to eat, she'd be yawning as I carried her down stairs. My belief is that that she's yawning to help her wake up - even I do that sometimes in the morning! I'll feed her and then do activity time until she starts showing tired signs again, usually its not for the expected 2hrs (she's 4.5months).... but if she started to show tired signs earlier, I'd take the cue and put her to bed early.
HTH!