So here's the rundown.

I look at a mirror, I seem okay. Not too chubby, presentable even. Clothes fit fairly well, posture is good and Don't look to shabby right?

Then, I take another look somewhere else. Half an hour, an hour later, and I'm shocked. I look round and rotund, and my face's gone flabby and my eyes beady and my hair looks like a cat died on my head.

Then, when I go home, I look in the mirror and I appear fine again.

What's your views on this? Should I go see a shrink?

From my analysis, you have low self-esteem in public. Something about being out of your house is making you feel inadequate. Given how we're bombarded by the media about beauty and perfection for both men and women, this is pretty common.

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The ultimate lesson of psychoanalysis is that human life is never "just life": we are possessed by the strange drive to enjoy life in excess, attached to a surplus which derails the ordinary run of things.
-Slavoj Zizek

Same happens with me. Weird, lol.
At school I look weird, tired and unkempt, bad posture, terrible clothing and so on. Overally I look grey and weak.
At home, I'm like a God of beauty and power in the mirror. (Though I might be exaggerating.)

Punchfacer: Glad to know I'm not the only one.

ConfidenceMatters: That's an interesting take on it. What do you suggest we do to correct this frankly embarrassing problem.

Actually, I think it's one of these:
1. You actually do look worse, because of the change in your posture when you meet other people, this could point to what status you perceive yourself to be and/or your social identity.
2. Being afraid, as in, seeing danger (being preventive of mistakes therefore seeing more of them) in social situations and feeling safe at home.
3. Comparing yourself to your surroundings.
4. You feel weak or lacking something.

I went to school today and looked at myself in the mirror, I look redic awesome. Felt strong too.

Reading my last post in my self-dev post (in the Lay Reports) might be helpful, I'm not sure, though. Try.

It could be the mirror. Before you get into the psychological aspects of it check out your mirror carefully.

I've noticed a lot of mirrors will be somewhat concave or convex. If this is because of the heat or what I really don't know but it makes a huge difference in how big or small you look. You wouldn't notice unless you look around the very edges of the mirror and check the background reflection to see if it's curved.