It does sounds like your burst disc. You can get them for about £3 - £5 depending on where you buy them from. Let all the air out of your bottle, put the bottle on your gun and dry fire it until it runs out of air, once your gun wont fire any more take the bottle off and press down the little valve on top of reg to make sure all the air is out. It should be relatively easy to push down, if it does'nt the bottle still has too much pressure in it. Unscrew old burst disc and replace with a new one. Make a note of what pressure the burst disc is so you get the correct replacement and screw it back on making sure it's screwed in tight.
Hope this helps. If your not confident doing it yourself get someone who knows what there doing, watch them, and at least you'll know what to do next time
Hope this helps. If your not confident doing it yourself get someone who knows what there doing, watch them, and at least you'll know what to do next time