that does sound pretty normal to me - if I'm in double figures it takes at least 90 minutes before a correction dose starts having any impact on my blood sugar & if I was rising when I injected it will carry on rising until it starts working - when you are high it just takes a while for it to start working.
Do you know what your usual correction factor is (how many units it takes to bring you down one)? What sort of insulin did you inject, was it fast acting bolus?
20 units does sound like a brave correction dose to me, so do make sure you are testing frequently, have a bit of patience to give up your correction time to work (it will take longer than you want it to, but once it does start working it might drop you like a stone), drink lots of water & test for ketones if you can.