Of course it's possible that a protein migrates at an higher molecular
weight than expected. The most wellknown example is p53 for which the
theorical molecular weight is something around 43 kD. It depends mainly
of the amino acid structure: with a lot of prolines the migration is
slower than expected. I'm now working on a protein that migrates at 190
kD but its calculated molecular weight is 135kD.
Be optimistic with your data.