It is normal for most dogs to lose their coat after having puppies. It is more noticeable on a breed like Samoyeds, they will lose almost all their hair and look quite awful. With my Samoyeds, undercoat starts coming out when the pups are 5-8 weeks old. Mostly gone by 8 weeks. Then they start losing their guard hairs, that happens when the pups are 9-14 weeks or so.
The coat on a Boxer would be the equivalent of the guard hairs on a Samoyed, so your dog is right on schedule. It might get worse. But don't worry, it will start growing back soon.
This is perfectly normal, not a sign of any problem. Most female dogs shed about four months after each heat cycle. When they have puppies, that shed is just much more dramatic.