Snake health questions

[kmj]

I've mentioned before that my current corn snake is my first so I'm going through some "snake stuff" for the first time, unaware of what is normal and what is not. Hoping to get some input before I take her to the vet (if that needs to be done.)

First of all, I'm not entirely sure she is a she. She's 2 years old but I have never got her sexed, I only looked at her tail and while I thought she was female, the bigger she gets, the more her tail is starting to look like she's a boy. I'm not sure if this is important or not but I'll have the vet figure this out.

She hasn't eaten for 3 weeks. Of course I panicked and went online after she refused the first time and apparently hunger strikes in spring is common in corns because of breeding season. I keep trying to entice her with pinkies every few days but 3 weeks later, nothing. Today she popped out of her bedding and her neck looked a little concave. Even some of her stomach. Not grossly concave but it definitely dips in, when it usually does not. She is drinking almost constantly, nothing unusual going on in her mouth/breathing, she is active, curious and her tongue is going crazy. Is this normal due to lack of eating? Should I get her to a vet right away? Other than that she seems like a healthy girl. Thanks in advance.