my son does this too, but in our case it is sleepiness. When I think it is just a waking yawn and wait a bit to see the next one, it never comes - the overtiredness sets in and the crying starts. In our case, DS can usually handle 45 mins - he is 8 weeks - of A time, so if it happens at 30 mins, it usually tells me his previous nap may have been less restful for one reason, or that he is having a sleepier day.