Coolant Pressure

Ok tonight after my truck has been sitting for 4 days I was checking things out for tommorow and noticed that my coolant line on the top of the radiator felt presured up. So I checked the coolant and as I opened the rad. cap it did indeed have quite a bit of psi. Is this normal? Have I not noticed it before? I don't have any coolant comeing out the overflow line. I have did the bottle test and get no bubbles on the overflow line either. I just thought after sitting for that many days and not beeing ran the psi. wouldn't have been built up like that. I have had this engine overhauled and the counterbores cut I would hope I am not heading down that road again. Oh ya and the aircompressor has less than 140,000 miles on it. The engine and upgrades are listed in my sig. Thanks