Just saw a picture of my ex and I feel like my stomach is burning and my lungs fell of my chest

We have been no contact for 45 days, and we stopped following each other on Instagram to avoid seeing each other and suffering. I just saw a post from a group at our university (we have different careers) and he is sitting in the front row. It's just his back, but I would recognize this man from MILES AWAY. idk I wasn't expecting this pain. If I start crying, I won't stop until I'm dead.