Replying to Ligament sprain?

So awhile ago in the fall I took a fall on the trampoline and injured my knee and it swelled up pretty nicely so I ended up going to the hospital and they said it could just be a sprain but they weren't sure so I got x rays and nothing showed up but I never got and mri die to the cost and thought it was a sprain. So if any of you have sprained your acl or mcl before what exactly happened? Did it swell up? I'm wondering because I want to make sure it wasn't anything worse like a partial tear.