What causes blood in the drain after a Tummy tuck?

It is very common to have some bloody drainage in a drain.  It is unlikely related to the pain you were having.  You are very early after a tummy tuck so these signs and symptoms are expected.  

Real blood clots in will not come through a drain. The protein of serum can congeal in the presence of red blood cells to make it look like it is true blood but often times it is not. Nevertheless report this to your surgeon.

You are still fairly recent in your recovery, and it is possible to have a blood vessel bleed on a delayed basis, especially if it was associated with a sudden change, such as the pain you describe.  It is also possible for positional changes to cause more blood to drain if it had pooled.  At only 10cc, you likely will be fine, but alerting your plastic surgeon and asking his/her advice is the right answer, especially if the bleeding does not quickly taper off!  Best of luck with your continued recovery!