While Crossfit is intense, it's not really the same kind of aerobic work required on a bike. So it might just be a getting back into it thing.
However, I have to say that something doesn't seem right here and I would have to agree with Oak, you should talk to your doc about it. In fact, if you were a client of mine, I would insist on it.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS