Could his shed be early? My BP sometimes 'blushes' when he's excited like when its chow time or when I hold him. Is it a constant pink? Is it getting darker? Is it the whole belly or just a single area? Could there be an irritant on his skin- like a cleaner or something with his substrate.
Everyone says to double check the husbandry but it sounds like thats not an issue. I'm not sure what it could be- I'm just throwing out Ideas. One of the admin may have a better idea.
Hope all goes well!