First off, it might just be some pressure issues caused by the backpack. I got carpal tunnel in one hand just by replacing a screen in a door! So when I got Peripheral Neuropathy [PN] at first I'd wonderred what I'd done? Turned out to be something complete different!

Not everything leads to MS! Or brain tumors! There are hundreds of neuro and spinal issues that can cause the numbness you are describing...with hundreds more sub-sets of specific conditions in each category.

The MRI is a good solid first step! To rule out spinal/structural issues... Here is an web site [click on the blue] that explains what a neuro looks at to 'eliminate' the testable causes and then get into the more subjective causes. http://www.neuroexam.com/content.php?p=2
Soo, get ready for a round of MRI's CT's and blood work...not to mention nerve conduction studies [necessary but bothersome for many]. This is all part and parcel of the whole neruo diagnosis process! How far you have to be tested depends on the test results of each step along the way. I truly hope you have a short series of steps and the whole thing goes away SOON!
Remember that there is no question too simple or silly! I've asked my share of them, I guarantee you! So don't be shy, please - j