@sad-crepe
Don't stick plasters on your nipples. You could try a square of gauze, placing that over your nipple, then use some kind of medically suitable tape (test if you're allergic to it first if need be) in two parallel strips to hold it in place while also giving the piercing maximum airflow so it heals properly.
I'm a bit surprised that any piercing establishment worth a damn would apparently completely seal a pierced area in a plaster as it's not conducive to long-term healing as air is required in order to let the skin breathe and the wound to stay reasonably dry.
I wouldn't recommend showering either. Have a wash, keep the washing away from your nipples and also - at least hopefully - the piercer gave you guidance on how and when to wash the piercings. I've had a pierced nipple before and it was long enough ago that I don't remember the initial instructions well enough.
If your piercer really didn't bother to tell you how to take care of the piercings to stop them going super-funky, I'd suggest getting some recommendations for a decent piercer and get advice from them. Nipples are a highly stressed area because common body movements will move and stretch the skin, as well as the clothes you wear brushing against them (wear loose clothing and breathable materials such as cotton).
You've basically got two open wounds with foreign objects stuck in them, the body doesn't like that very much. If you don't manage the healing process correctly, you'll have an infection in no time.