My wife's just had something similar. She went to the doc's who told her to go to A&E (because the doctors didn't have one of those massive magnifying glass things which they use to look at your eyes).
After waiting about four hours (!) she was seen and told that she had an infection on the white of her eye. She was prescribed antibacterial eye drops which seem to be doing the trick. As well as the gel-filled blister, her eye was very bloodshot around it. Yours might not be the same thing, but if it doesn't go in a few days I'd advise you to see your doctor.