Our son got tubes a little over a year ago and did the same thing for a while. He had chronic ear infections for the first 10 months of his life. Since he's had the tubes, he's only had 2...and they were very minor in comparison. For almost 6 months after he had his surgery, he would pull on his ears, pat them, etc... We were sure they were bothering him, but the doctor would check them and tell us everything was in place and looked good. I think that they just become more sensitive with their ears after the surgery - not sound-wise, but touch-wise. Hayden is now 22 months and he doesn't pull on his ears anymore, but it is still impossible to keep his wax ear plugs in during bath time, and getting a haircut always gets a bit crazy when they cut around his ears.
When in doubt, just take him back to the doctor and ease your mind. Those little suckers (tubes) can slip out of place pretty easily from what I've heard. I've had about 8 false-alarm ear infection visits in the last year :)
That may not help much, but at least you know it's common. Hang in there!