To feel a tiny bit offended he wouldn’t sit next to me.

[OnAgainOffAgain]

My department is split into categories and every quarter we have to present to the whole department on what projects we’re working on. And today was my first time speaking as I’m still relatively new.

I’m in a team of 3 people, one of my colleagues said we should sit together (we sit around a U shaped table and no one stands up) as it’ll be easier to present. We walked in and we sit together, colleague then said to manager “hey sit next to OnAgainOffAgain” and he obviously heard but still sat away from me.

Just feel a bit self-conscious now

Were all the other teams sitting together?

I'm unsure if it was the manager of the whole department or one third of your mini team. If you were presenting together it would be usual for the three of you to sit together. If you were presenting on behalf of your trio, not so much. If it was the manager of all of you I'd assume he intended to 'chair' the meeting and had already mentally chosen his seat.
Had I been anyone in this scenario, so you, colleague or manager it would not have crossed my mind that this had anything to do with you. I'd imagine it just came down to seat position.
BTW, don't second guess his reasoning. I like to be near the door. I dislike U formations and will stand rather than twist to watch a presentation. I'd also rather look to my right than my left and don't want someone behind me. If air con is on I sit underneath, if heating is on I seek a cool spot. I don't wanna sit facing the sun or a rattly blind, and i dont want to sit next to smokers or perfume....I could go on.

^
This.
Plus doesn't want to be overlooked so he can surf the internet while other people present
Wants to put himself in the direct eye line of someone he wants to impress/suck up to

Rude if you were presenting as a team though.

You're over thinking.
He may have fluffed or eaten onions and was self conscious.
Brush it off and start afresh tomorrow🌻