is this a blown head gasket ?

It's perfectly normal for pressure to build up after the engine warms up because coolant expands as heat is applied. It is also normal that if you remove a radiator cap from a hot engine to have it spew the antifreeze out so it's not a good idea at all to do so. I've seen guys get major burns doing that. If your car runs good and doesn't loose coolant you have nothing wrong. If it builds too much pressure it'll puke it out of the overflow hose/tank. That's when it's time to check things.