I'd go to your GP - ask for a second opinion. If he/she has any doubt they will gladly sign another x-ray request for. Hospitals don't like to do 2nd x-rays as they are not good for you (my sis is a radiographer) - but as your thumb was swollen, they could easily have missed something.
Case in point: I went to A+E with an injured foot only to be told it was a sprained ankle. I knew something was wrong so I went back again - only to be told to take painkillers and go away. My sister told me to go to my GP who was concerned and demanded another x-ray. This time it showed a fractured metatarsal. The lesson? be persistent, it's your body and you could get arthritis at a later stage....
Good lick :o.