I could see hitting 185 running no problem. I probably hit that with a sprint. On a bike though? I'd probably fall off before I could get my heart that high. Are you gearing right? Do you have heart issues?
I dunno, I wouldn't let that get in the way of me getting ready for that ride you had, but it is a warning sign to be careful. Report the avg. change during your training period, that could help. Also, do you know what the error rate is on your HRM?