It sounds to me like your medication is doing its job to me. If you woke up to this is could be a number of things,,,,alot of things happen to us when we sleep which primarily could be believe it or not bad dreams that can cause our hearts to race when we sleep if your blood pressure was 128/79 that in itself is not alarming and if your pulse was 127 well that is a little high but not crazy high. I agree with Jerry i don't think that it sounds too scary but if you start having repeated episodes and it looks like your pulse in going higher and higher during each episode you may want to give your doctor a call. You did have knee replacement surgery which is no little deal and includes a period of time that your system has to get rid of all of the meds that they put into you to put you to sleep that sometimes can actually take months for your system to rid them not to mention that your body is adjusting to the fact that you have a foreign object in it that it does not recognize and will try to compensate for in many ways. If in doubt call your doc but it sounds like your meds are def. working as your safety net which is what they are intended to do.....good luck
In the absence of any ongoing symptoms I think it would be best to put the episode out of mind. If it comes back again when you are not thinking/worrying about it, it may be time to talk with your doctor. That's what I'd do.