You probably sound obnoxious in your writing

It's probably took him too long to look up all the big words

I'm going to say you deserve it here for not taking some valid criticism to heart. You don't need to use the most complex words you can all the time to sound more intelligent. Sometimes its better to keep it simple and get to the point. There is a time and place for an eloquent vocabulary, but overdoing that will often make you sound pompous, or even make it seem that you don't really know what you're talking about.

...and what's up with the word: 'monosyllabic '? Huh??

A lot of my students use unnecessary long words, I suppose to try to sound smart. Their work usually ends up being overly convoluted or just incomprehensible. They also usually use their chosen long words incorrectly. I usually tell them something similar to what your teacher said, though I try to be more clear.

"Never use a long word where a short one will suffice." -Orwell

Well it sounds like being an over achiever really worked out for you. Haha.

Nothing spells "America" better than arrogant state accredited university students who are too far up their own to realise when their professor gives them some valuable advice... Just sayin.

I agree with the general opinion here, but I don't think it's anybody's fault. Everyone will reach a point in their educational career where you go from 'learning how to use big words' to 'learning not to use big words'. For some people, that moment comes when they get their Literature degree and fill out their application form at McDonalds. It's encouraging to see that the OP and most commenters have learned that lesson earlier.

Sorry bout that mate, still, knowledge is power. So don't beat yourself up over it ;)