we just bought a used Free Flow portable spa. for some reason after filling when plugged in the jets started. it has a single pressure switch for the jets and I have checked and the switch seems to be working normally (I can hear the switch click when you press the pressure switch. Could this be part of the heat up function of the tub?

Yes, it could be part of the heating cycle. If you don't have a dedicated circulation pump for your heater, your main pump should turn on (low speed) until the unit reaches temperature. When you cycle your pump switch, does it cycle from low to high? If so I would suspect everything is working as normal, so just put it back on low speed until it starts heating. You may find that you have 2 or more low speed settings and one high right now. When the tub is at temp, one of those low speed settings should changed to off, depending on the type of switch you have in there.

Enjoy!