I have a twin sized and she wants a twin sized.

She has a full sized and I want a full sized.

Should we just switch or buy new mattresses because that's weird?

I think it's fine if you put your own sheets on. Just make sure the mattress she's giving you isn't old and needs to be replaced soon anyways

As long as you think her home is clean and doesn't have bedbugs, fleas, or anything, you should be fine. Switching is fine, if it suits you both.

I have a twin sized and she wants a twin sized.

She has a full sized and I want a full sized.

Should we just switch or buy new mattresses because that's weird?