My husband was away and after getting a positive opk on Saturday am we didn't have sex until Sunday night - is that too late to get pregnant?

Not necessarily. Getting a positive on a opk typically means that you most likely will be ovulating within the next couple of days. You may have been right on time, or you may have missed it. Either way, don't stress out about it. Otherwise it will make it more difficult. I had to have a minor surgical procedure and wasn't allowed intercourse during the time I was supposed to be ovulating. My dh and I had been trying for about 6 months, and thought we would be going into month 7. I actually got pregnant that month! I wasn't stressing about it and was just enjoying myself. Two u/s later confirmed that I had conceived very shortly before my period was due. Never give up hope, and always enjoy trying! This could still be your month! Baby dust