I did it!!! but now a problem

I have been playing around with the idea of replacing the valve stem seals on my 560 powered 82' 380 SL for awhile and with all the help I got from many of you guys I finally did it. Started it up and a little smoke and then it quit smoking and I took it for a test drive and all seemed well, no smoke and boy was I happy. Then
I noticed after only about 8 blocks or so my temp guage was close to 120. This distance was short for a car started cold. This has not been a previous problem. I came home and checked the oil and coolant and all was at proper level. Is there something that I could have done that would have caused this. Any suggestions would be appreciated.