My DD has done it. ONe of the bubs at Mothers Group did it the other week and his Mum rushed him to my place to check over.
Panadol can ease the pain which might have been stopping her from sleeping. How long between the bump and getting drowsy? If it is her normal sleep time, I would be less inclined to panic - just make sure she is easily rousable. I would still get her checked out by the doc if you are concerned.

HTH

MG