If you had a feel then its a good call and I tend to go with my reads.  I'd rather lose money following my instinct than ignore my instinct, and if I am wrong, at least I will learn something.  On this specific hand, Seikei could have called with anything as a “BB special” bc you raised so low.  Typically with a mid pocket pair I like to raise more pre.  That aside, his lead out screams like he has a 5 or 7 to me.  If he had a 10, a set, or a straight I think he would check-raise.  On the other hand, I'm not sure he'd lead out as a stone bluff with two players in the hand.  To me, it seems like he has a weak pair, and just wants everyone to fold.  When he checks the turn, it seems as if it is with the intention of check-calling so that confirms my read.  However, since you bet so small on the turn him calling doesn't really give you any information.  When he leads out on the river though, keep in mind that some villians will lead out when they have a big hand, and aren't sure that you are betting yourself.  I have to be somewhat worried that he binked a 5 or backdoored a straight with an 8 in his hand.  The pot odds are 2:1 so if you think he's bluffing 33% then its a good call.  Granted, I would have played 99 differently from the get-go.

Phoebus said:

I should have bet bigger on turn. Idk about changing my raise size depending on my hand though, i'll get torn apart when I move up if I do that.

You shouldn't change your raise size based on specific hands, but that doesn't mean you should always bet the same amount.  If anything you should be randomizing your raises, sometimes min-raising, sometimes 2.5BB, sometimes 3BB.  That keeps your opponents guessing more than constantly raising the same amount imo.  It also allows you to raise 3BB here, which I think would have been more optimal.