Volvo Repair: xc 70 cross country temp guage in red area, radiator hoses, cold engine

Question
It is 92 degrees outside; just had AC recharged; today; now temp guage is going up to top; red area; after even just 2 miles from cold engine. Then fan is on constantly; if wait, can go 1-2 miles again before guage is at top; but no steam or overflow.
What is cause?

Thermostat? If so how do  find it in radiator hoses and get part and repair self. Can I buy part at any auto store?
As usual problem happens on weekend.

Answer
Peter, can be the thermostat.  While you are there, I would change the ect sensor which is on the Thermostat housing. I wouldnt buy those parts at a auto parts joint.  Stick with Volvo sensors and Tstats.  Fan being on while A/C is on is normal.  Roger