Boxer bitch being aggressive

Question
I have had my boxer bitch for over a y she is just over 2.not spayed due into heat soon. We adopted a older male boxer who is spayed very gentle giant we have had him 5  months took time for them to get on but they get on brill except feeding time. This week she has gone for him 3 times quite aggressively I was wondering if this is because she is coming into heat .

Answer
Your question is confusing to me.
Feed them at separate locations apart to stop the aggression.
She is exerting her alpha position within the household because she was the only dog before and considers herself the alpha and he the follower of the pack.