Personally, I would understand what you meant, albeit odd.
Because the most common definition of freeze is, "to become congealed into ice by cold", we associate the words freeze, freezing, and frozen with cold or ice.
However, in nearly every action movie with a cop, if the cop is wanting the criminal to stop moving, they yell, "Freeze!", where here it means, "to become fixed or motionless".
So grammatically, you are correct. However, because you're using the word in an uncommon manner, it leads to confusion. Especially since "to freeze", refers to the idea that you have actually frozen something into ice, "freezing a semester" seems unusual, as a semester isn't a physical object that can be made cold, or frozen.
Synonyms for what you're looking for include: paused, ceased, and suspended.
However, semester itself isn't something that someone typically has control over (which can add another layer of confusion). So saying that you've frozen, or stopped, a semester, means that you used your god-like powers to prematurely end the semester.
You might be better off just saying, "Since the semester ended, I didn't have much going on.", or "I didn't have much going on, after the semester ended".