Concussion CAN be a very serious injury indeed especially in the circumstances you are under. Any head injury should be monitored for 48 hours, any periods of dizziness, sickness, tiredness or other symptoms should be noted and watched with care. Concussion can come minutes, hours or days later. Similar to the secondary drowning theory, you can suddenly collapse or similar. Not that I am trying to scare you, but concussion is very serious, as with all head injuries.
I assume though if A+E gave you the all clear you should be fine, however as I said you should keep an eye on it.
You could also have been suffering from a migraine, these can be set off by head injuries, the symptoms you described sounded like a migraine. The best advice is to sleep it off but make sure someone knows as migraines can turn nasty.
As with anything, you probably will feel the jitters going back into the net, same with car crashes and other injuries, perhaps wear a helmet until you feel more comfortable.