Mucosal thickening in the Maxillary Sinus is usually due to chronic inflammation. When you blow your nose forcefully, you may rupture a few capillary walls especially when you have a nasal septal deviation with a sharp spur.
Next Steps
If you do have a nasal septal deviation with a spur as I suspected, please undergo Endoscopic Nasal Surgery followed by a few months of intranasal corticosteroid sprays to heal the chronically inflamed sinus mucosa.
Health Tips
Find a good ENT Surgeon in your city to help you out.