Alarm problem

[Big Col]

Hello all, The alarm seems to be playing up basically it wouldn't let me in it this afternoon. When I tried to unlock it with the key in the door lock the alarm went off, I put the key in the ignition and turned on it went off but only the drivers door open. I noticed the red light on the dash which normally flashes quite fast was flashing like a bloody strobe light then after a while went back to normal flash. Still can't open other doors its driving me nuts.
Any ideas?

Regards Colin