I've had two budgies for a bit over a week now. The female one sometimes tweets a lot. She started tweeting yesterday and today in the morning after I removed the cage cover, might have done that before too but I can't recall. The tweets are pretty consistent (sound the same) and there are a couple seconds between them. She has been tweeting for about half an hour now. A couple times the male answered with a similar but quieter sound. Do these sounds signal that she's upset about something or what do they mean? What could she be upset about?

She might just be singing to herself? It depends on what sound she is making (as different tweets or chirps mean different things). Budgies like to sing or tweet to themselves to comfort themselves and each other,  to say there is no danger. I think the chirping is fine,  budgies are naturally loud and like to chirp to each other.

Okay thank you, she just sounds always angry compared to my male budgie but I guess that's common.

There is a distinct way they chirp when they are angry. There is one loud chirp as if its calling out 'hello anybody around' and its kinda ringy on the ears. They make it when hear other budgies chirp or a similar sound. Also there is an angry chirp that is more sounding like they are sounding the letter 'ch' over and over. They make that sound also when they don't want to be touched. Mine make it when they are feeling  cocky or naughty. I love to mimic it back at them and they fly angrily over my head.