Insulation has changed air flow pressure

Our 1971 built house had barely any insulation in the attic. We moved in and added new batts of fiberglass insulation and foam sealed crevices. Ever since then there has been negative pressure in our house. According to the manometer -0.02 reached with all doors and windows closed with 2 exhaust fans running. I’ve been suffering from sinus swelling and ear pressure ever since and it’s been miserable to exist in there. I’ve checked myself medically and had extensive ENT and allergy tests done which all come back clean. HVAC people consulted and suggest adding mechanical fresh air ventilation.

I’m wondering if anyone out there can explain why this might be happening? Our house is old so I’d expect for it to still breathe, but apparently it’s not. Door blower test done and the house wasn’t deemed too tight, yet negative pressure is for sure happening. No visible ducts have leakage. Could attic insulation and some foam sealing really cause this caliber of issues?