It's not the "kissed" part that bothers me. It's the "made up" part. When you "made up" with your ex, were the two of you "getting back together?" I mean, is this guy floating home thinking that he's getting another shot? Does he think that the two of you are a couple again? As in, two people who are exclusively seeing one another?

The devil is in the details. And the devil's got some sharp horns.

I would clarify your relationship with your ex/no ex. The fact that you're interested in another guy suggests that you might not be ready to get back with your former flame. See, this is the sort of predicament that guys find themselves in, and this is why we get such a bad rap. You want to keep kissing your ex, and enjoying the familiarity and comfort that brings. You also want to get with this other dude. You want both, but you only get to have one. Unless you want to be a lady douche.

So... be honest with yourself. Ask your ex what "making up" means. If he thinks it means the two of you are dating again, then I wouldn't risk your romantic karma by, essentially, dogging around behind his back. Maybe you and your ex are "seeing other people." This would mean you can do what you want until one of you cries Uncle and either wants a relationship upgrade or a one-way ticket back to singleburg. If your ex wants to date exclusively again, and you don't, then don't lie. You'll be doing neither him nor you a favor.