Losing weight and motivation due to stress at Uni: Feeling overwhelmed

Have you looked into the symptoms for things like depression? Uni is a pretty common time to develop things like depression because of the complete change to life and added pressure of studying compared to college. You can check out some info and advice on depression and other mh issues on sites like mind.org and sane.org. If you think it sounds like you may be affected you can go see your doctor and also talk to your uni student support to see how they can help you. Unis will usually have a free counselling-like service for students having difficulties and they may also be able to help ease the pressure from uni a bit.

It may also be worth looking into things like anaemia or other deficiencies if your diet has changed since going to uni (which it usually does) almost all deficiencies have symptoms of low motivation, weight change and feeling run down and they can also contribute to mental stress and feeling overwhelmed.
Again this is a talk to your doctor thing and they can run some blood tests.

Whatever the issues is and even if you don't work out any cause you can have a chat with uni about how to help you cope better and not feel so overwhelmed. It's a more common feeling at uni than you might expect so yours should be used to dealing with it by now.