My back hurts

I'm a 26 female I'm 6 feet 2 and I weigh 423 and my lower back hurts it's fine when sitting but when I bend over or start doing stuff it bugs but tonight it really hurts is it because I'm fat ?

Very possible. Before I started loosing weight I would sporadically have low back pain, sometimes to the point where it would go out for several hours.

And for a long period of time, my sacroiliac joint would go out and I would have to go to physical therapy for it.

So far since I've lost I've had no issues.

It very well could be.  Before my surgery and little bit of weight loss I had sciatica.  Now, after only losing 29 lbs, it is gone!