I was loading line on my shimano spinning reel today and for some reason the line was overfilling on the side closest to me and refusing to fill up close to the lip.

What can I do?

It could be that the spool is not seated down on the post all

the way. Check the underside and see if there's debris inside

of it, preventing it from being seated properly.

This would cause the spool to spend more time in the up position

when rotating, hence more line at the bottom of the spool.

Hope this helps !!

I'd strip the line off and try again making sure you have the rod in a "fighting" position, reel the line on slow and methodical.  

Some shimano models come with or a shim or can be obtained, but I would post this in the shimano help section and I'm sure bantam will have the correct answer.

Depending on the model,theres usually 3 washers/spacers(dark grey) which sit up under the spool on the shaft.Take 1 out,leave other 2 should go on nice and level for you,dab of grease.G/L

To adjust how the spool fills add or remove 1 or 2 the dark gray fiber washers on the shaft under the spool . The manual details the procedure.

I was loading line on my shimano spinning reel today and for some reason the line was overfilling on the side closest to me and refusing to fill up close to the lip.

What can I do?

I agree with the comments about the washers. The only thing I would add is before you do any adjustments, if you have the opportunity, is to hit the water, any water, get a lure tied on and cast and retrieve it under normal lure resistance to see if it spools any differently from when you do it in your basement or wherever.