Female in Heat
I'm sure that just as in people, all dogs can develope differently. But, female boxers don't develope fully until their at least 2 years old. Until that time they shouldn't breed..early breeding can cause alot of serious health issues to her.
In relation to your concerns...our female is just now turning 2 years...she's 28 months now. She didn't have her first heat until she was about 16 months old. It lasted about 4-6 weeks (I can't remember exactly). But we also became concerned that she may have become pregnant for the same reasons you mentioned - staying swollen, nipples slightly larger, etc. We were told that was part of her changing body. Kinda (but not exactly) like when a young girl goes through puberty

Anyway..we were worried, but it turned out she wasn't pregnant...just normal changes. She had her 2nd heat this past Christmas, and is due for her 3rd anytime now.
We put a doggy diaper on her, that helps alot. During that time it's always obvious to us (with people we come in contact with) just who's a doggy person and who isn't LOL. Those who aren't doggy people laugh at the diaper and think we're crazy...those who are understand.