Receiving gifts or surprises of any kind from anyone makes me extremely anxious. 

Not because I fear I'll be disappointed and won't know what to say or feel that I don't deserve it but more so because expressing excitement/surprise/happiness around others is difficult and makes me go into a sort of panic.  

For most people around me, the experience of receiving a gift seems to be nothing but pleasant (or if not, for understandable reasons) so I feel really weird for having this problem. 

I'm scared that if I don't express enough positive emotions in that moment, people will think that something is wrong, that I didn't like it (which is hardly ever the case) or otherwise misunderstand it. 

Even when I am genuinely happy/surprised, I keep finding myself trying to force an over-the-top reaction of "woah!!!", after a couple seconds of which I feel intense anxiety and a need to hide away so that I can take a break because I feel as though I just can't carry on with the emotion anymore. 
I try to casually take attention off of myself in whatever way possible so that no one notices how overwhelmed I am. 

I don't know why or when exactly I started doing this but it's been an issue for most of my life, so much so that I get anxious for special occasions several days in advance. 

It was the worst when I was younger because I knew kids generally express more excitement in these situations and that's the normal reaction that would be expected from me.
So I felt I had to keep up, not to mention that it'd be strange if I started behaving differently one year all of a sudden. 
(Coming back to the point about fearing misunderstanding.)

Even seeing other people react to gifts gives me a feeling of anxiety and secondhand embarrassment because I keep picturing myself in their place. 

I don't know why I can't just express what comes naturally and what the root cause of this is.

Does anyone else have this issue? 

I'd love to hear about other people's experiences because currently I feel pretty alone in this.
Thank you in advance