I'm not even Catholic (yet). I'm going through RCIA and may convert in the spring... so you can take or leave my opinion.:)
I would say if your arousal was a surprise and a bodily reaction to something your boyfriend did which was NOT intended to arouse you at all, I think you did not sin. If you lingered on the arousal, wished for more, fed your desires, etc, I would say yes, you sinned. Otherwise, I don't think so.
Did your boyfriend sin? Maybe. I'm not sure whether you mean he was rubbing your lower belly on your* skin*, under your shirt or pants, or what. If he was rubbing your skin, my opinion is that is inappropriate affection for an unmarried couple and he should not do it again. Even if it was not on your skin, the fact that it did arouse you tells you you need to talk about where is OK to touch and where is off-limits and make sure it doesn't happen again.
Even though I'm not Catholic, I have looked up the definitions of different sins and I believe in order for a sin to be "mortal" it has to meet certain criteria:
1) it has to be a "grave matter" (the act is intrinsically evil... murder, rape, incest, adultery, fornication).
2) it has to be committed in "full knowledge" (The person must know what they're doing or planning to do is evil and immoral.
3) it has to be committed with "deliberate consent" (The person must freely choose to commit the act or plan to do it. If they are forced, they have not committed a mortal sin.)
I think clearly your situation does not meet any of these requirements (and even if it only does not meet ONE, it is not a moral sin). Hopefully that makes you feel better. I think if anyone sinned, it may have been your boyfriend, and definitely not mortally.
I think it is fine and good to bond through affection being unmarried, as long as it is non-sexual affection (like stroking an arm to make someone feel better, or tell them you love them), a brush of the cheek, a hug, etc. Obviously (to me anyway) these things are healthy and fine for an unmarried couple. But any shows of affection that you would be uncomfortable doing in a restaurant or in public is probably inappropriate. I think that is a good rule of thumb.