My Shih Tzu Hasn't Been in Heat.Help?

Hi! I have a female 2 Year old Shih Tzu Named Bella. She has brown and white markings and she is pretty healthy and playful. the last time she was in heat was around her birthday, March 2nd but now its June! She weighs about 12-20 pounds depending. She is eating and drinking and pees and poops just like any other dog. She also still humps my arm. I'm pretty sure she is not pregnant because she is our only dog and we watch her as she pees and poops in our backyard. Our backyard is fenced by the way... Also, we have never lost her and when she goes for walks there are ussually 2 people with her. And she's leashed. I have no ideo what happened to her but all I know is---Bella, my shih tzu is still humping my arm and she hasn't been bleeding thankks!