There are 4 reasons that I can think of that you may be having this problem:
-Stirrups too long (already mentioned)
-Not enough weight in your heels
-Horse not carrying himself and making you work too hard and in the process you are drawing your leg up or you draw your leg up because you are not balanced enough which could happen if you are riding a new horse and you aren’t quite with the rhythm
-Stirrup iron too wide
Hope that helps.