Hi there!
Sweet potato isn't "poisonous" bunnies, but they are high in starch and makes it hard for the bunny to digest. Especially when introducing new food into a rabbits diet, you need to ease it to them slowly, by increasing their intake daily. Foods that have high sugar contents though, are not great for rabbits, so they should only be given in moderation.
You can check out this site for more information about rabbit diets before you buy any more food!
https://www.naturalrabbitfood.com/can-rabbits-eat-potatoes-potato-peels-sweet-potatoes/
Another good site to read is found on the House Rabbit Society pages, which you can find here:
http://rabbit.org/suggested-vegetables-and-fruits-for-a-rabbit-diet/
Most likely, your bunny just need to poop out the undigested stuff and should be better! It also helps to know the sex of the rabbit as males are lazier than females at times, and tend to just lie around for long periods of nap time, from what I've noticed.
Make sure you're checking the poop - ie. if it comes out hard balls (normal) or soft cecotropes (in excess is odd because they usually eat the soft stuff to redigest it). Certain foods affect different bunnies in a different manner. I personally try not to give too much fruit to my bunny because he gets very lethargic after the initial excitement and he no longer wants to exercise or do anything.
If in a few days Nibs still doesn't have interest in the hay or pellets (which is crazy because my bunny loves pellets!) then you can try new and innovative ways of enticing him. Perhaps mixing a little bit of banana or apple in the hay, or putting the hay in a toilet paper roll, or even spraying apple juice on the hay will work!