Rachel, part of the problem with coming online for advice, esp with something very emotional and personal, is that the anonymity/facelessness of the net makes it hard to judge the credibility of the advice given, so it's easy to get more confused and upset by what you read.
FWIW, my sense is that if your uncle has always tickled you, this is his way of showing affection. There's nothing perverted or sexual about it. Both of you probably enjoyed this, I bet you gave him some really deep belly laughs! Who doesn't like to be tickled? Judge this action (and the relationship you have with him) in the broader context of the entire relationship. If he treats you well, is respectful of your boundaries (as in not staring at you or touching you at other times or other places), and generally 'normal' (as in like your folks), then what's changed here is puberty. Touching in general, and tickling in particular feels different now that you're in puberty, it's not just 'fun', but touching your body has a very sexual feeling to it, right? That's what you're calling 'weird'.
Your uncle might realize that your body is changing, but he might not realize that your feelings about it are changing, too. So, he's behaving the same way he always has with you. If you are uncomfortable with him tickling you, when he makes that move to tickle you, you should pull back, look at him, and very nicely, yet firmly, tell him not to do that anymore. Just like that.
Some things necessarily change as you grow up, and the way people express themselves to you is one of them. You and your uncle might need to find a more age appropriate way of expressing the affection you have with eachother in a way that isn't so 'weird' for you. This isn't such a big deal, and in the absence of any other kind of 'weirdness', your uncle is probably only guilty of a lack of awareness here.