Strong students can sometimes go through high school fine despite small (sometimes even large) disabilities, such as test anxiety, minor vision issues, etc. It's possible to work around this stuff in high school but suddenly everything is harder in college. So problems appear out of nowhere.
A typical pattern is that the minor problems slow you down, so that you run out of time on exams. Maybe just on harder exams, but probably in more than one course.
If this sounds like your situation, speak to the Counseling Center. They do screenings for a range of likely problems, and can identify whether you have any problem that would entitle you to extra time on exams. This process goes a bit slowly but an exam accommodation (if you get one) lasts for the rest of your time at UIUC.