Definitely see a doctor about that and try not to get so upset and fighting with your girlfriend. Sounds like your body was not ready to sit down. You were really wound up and sitting down had a sprung coil effect. Next time you get into such a uproar, try going out for a jog to release the bend up energy.
Also if i might add, if you see the conversation with your girlfriend not going anywhere positive, just try to walk away or change your approach to what you are saying.
You are one person and she is another. You both have different lifes experiences and perhaps different lifes outlook. She is not you, and you are not her. You are 2 completely different people, who, when its all said and done will be accountable to yourself in the end.
Opinions are not the issue, but ones ability to communicate and this communication, if its of love, should be set where the outcome is loving.
You definitely need to see your doctor.
Wow that sounds scary! I am not sure what it could be. I would say talk to a doctor about it.