Was she ever confirmed pregnant by a vet or even blood or urine test? Maybe she is just fat.
That said, she might have gotten pregnant, might have absorbed it, might have slipped/aborted it... might have gotten a cyst on her ovary and not cycled after that...
Agree that heifers are unpredictable about udder development.
If you are not comfortable with "arming her" to check inside, and cannot "bump" a calf by the above method.. I would suggest a vet check or an experienced neighbor that could use a glove and go in and arm her. This close to due date. the calf should be very obvious in most cases.
Due dates are approximations with a variation of about a week according to breed. According to the date you say she was AI'ed, Feb 15th-20 would be a reasonable expectation. I am thinking that she has lost it somehow and has gotten fat... hope that is NOT the case for you. I am not aware of any differences in mini's for average pregnancy length.