Seeds with my indoor tent grow

Im noticing lots of immature seeds on almost all of my ladies. I noticed a couple nanners during the last week of flowering on 1 plant but I doubt this was what caused the seeds.

I have a tent in a room that is kept cool using outside air. I place a fan in my window, which has a screen and is cracked about 3", drawing the air in. Logic tells me that I was probably drawing in pollen from outside. The grow started mid September in New England where we had plenty of times with pollen in the air. Is my hypothesis correct that I got my girls pollinated by outside air?

I don't mind a few seeds but I'd like to learn how they got to be with no males.

Thanks in advance.

Either is possible, however if you noticed nanners, that is your most likely male. To eliminate this problem you want to add a HEPA filter to your intake and fix the problem that lead to nanners in your grow room.