I've had my belly piercing for two months now and recently just changed the piercing, starting to get red and tender at the top of the piercing also had white/beige pus. Is it infected? Should I change the piercing back to the original one I had?

Pus is usually a sign of infection. I can't see that changing the jewellery will have much benefit, unless there is bacteria on the bar and it could do with a cleaning... But generally, playing around with a piercing that has been recently done, and changing or removing the bar increases rather than decreases the likelihood of infection.

What I would suggest is to wash it with a hot water and sea salt mixture. Do that regularly, ensuring you softly pat the area dry and keep it dry afterwards.

Heat and salt both act as an anti-bacterial agent, and bacteria also need moisture to reproduce.

If, after a few days of doing this, it still hasn't got better, I would recommend you visit your piercing studio or even your doctor for professional advice.