Why was he staring at my body?

I have this friend whois a guy. He is always respectful, nice, polite. He has never shown any sign of interest towards me and I though he saw me as a friend only, maybe like a sister. So I did not drink on that day last week, but he was drinking with his friends and after some time I saw that he was clearly staring at my body, especially my chest area and my belly. I was a bit embarrassed with the situationI wonder If It could mean he has some time of interest in me when he's sober? I don't think I will bring It up this situation with him, unless really necessary.
Why was he staring at my body?
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