I had this issue. I resolved it over time by getting my baby back to the breast. I would use a hand pump just to get letdown - literally, about 5-6 squeezes of the pump (it was unbelievable, I would get 2-3 ounces in that initial 5 pumps, no wonder the baby was choking). After those initial 5 pumps, I would put the baby on to nurse. He could nurse comfortably, with no choking, if I put him on after the forceful initial letdown was done. Then, I would nurse only on 1 side per feeding. Put him on to nurse after the pump for a few minutes, stop to burp, then put him back on the same side until he's full. On the other side I would use the hand pump and pump just enough to be comfortable (maybe 10 squeezes on the hand pump). Next feeding, so the same on the other side.
If you do this, your supply will eventually get back in line. In the meantime, you can freeze all this extra milk in case you need it later.
The hand pump works better for this, because it's too easy to over-pump using an electric pump.
For more info, look at kellymom.com, and search forceful letdown and oversupply, which I think are your 2 combined issues.
You can work this out! Good luck!