Oh, so no braces and no hardware? That kinda narrows down my thoughts. I assume you did have a genioplasty, correct? And they removed ALL the hardware from your entire face?
No, I have no idea if Botox would work or not. It depends on why you're experiencing the tightness you're having. It's got a pretty low risk profile, so I would probably give it a shot. Depending on what it's being used to treat, it's injected differently (or that's what I've been told), so I would see someone who has treated stuff like this before.
The only two things I can think of, that would be causing what you're experiencing, are scar tissue or angry soft tissue. Scar tissue is really difficult to treat. If it's horrible and there's a lot of it, they can sometimes perform surgery to remove it, but that comes with its own set of problems. Surgery itself creates scar tissue, so when they perform surgery to physically remove it (cut it out), more is actually formed by default. You could try acupuncture, or look into other complementary and/or alternative therapies. It also may just get better on its own, with time. Angry soft tissue is sometimes easier to treat. If your problems are muscular, then Botox has a good shot at helping. Sometimes just one treatment is enough to break the pain cycle. For soft tissue stuff, you could also look into acupuncture or other complementary and/or alternative therapies, too.
Does your chin burn at all?
