Dogs Aggression toward other dogs

Question
I own a yellow female lab who is a year and a half old now, but have had her since she was 8 weeks.  At home, around my husband and me she is quite calm, except of course right when we get home or when others come into our house, and rarely, if ever, barks.  The first 5 minutes or so when we get home she is wild, jumping and wanting attention, but then she calms down.  She is very sweet, loves being close to us, and is quite gentle when we play.  This afternoon though I took her to an off-lease dog park, which we have been to many times.  Normally she plays with all the other dogs very well, running after them, normal dog play.  Today though, another lab picked up a toy of hers and she went after it, growling, and the hair on her back was up.  The two dogs really got into it, and although neither was really hurt (thank gosh) the situation really scared me, like I can't trust her around other dogs, or kids for that matter.  Do you think the toy was to blame, that she was protecting it?  Should we not take toys to the dog park?  Does this indicate she is a mean dog?  I've never seen this side of her before, although I have noticed the hair on her back stand up before, but she acted playful the other times.  Any suggestions?

Answer
Stefanie, I apologize for the tardy reply. I work for a CPA and this time of year is horribly busy for me. You say you've noticed the hair stand up on her back before; could you tell me when and what was happening? Also, what did you do when she went after the other dog at the park? From what you described, she was being possessive over the toy. Was it a toy you had brought from home? If so, then I would recommend that you leave her toys at home when you take her to the park.