I'm sad my dog growled at me. I always treat her right why would she do that?

She's either sick or pregnant. Either way, time to see the vet...

My girlfriend's dog started growling at everybody (even my girlfriend) a few weeks before they figured out that she was pregnant with 7...

Whats the context? It could be because she's injured or sick. But also, animals have different moods too so she could have just felt irritated and wanted space

maybe she has something else going on that's not your fault, we all have bad days

Maybe not feeling well. Something might be hurting her.

She may not have been growling AT you, she could be warning you about something.

Maybe he/she is upset about something.

Here is a guide to help interpret:
https://www.purina.co.uk/articles/dogs/behaviour/understanding-dogs/what-does-dog-growling-mean

What prompted that?

That time of the month?

She’s not feeling well

Yes it is