Is it raised? Swollen? Warm to the touch? If so, then it could be an abscess. You injected 10 hours ago? How long after that did the bruise arrive? Most likely, you probably used too large of a gauge needle for your hands (only use something that is a 31 gauge, short tip [5/16"], ultrafine syringe for that...30G 1/2 inch can be a little rough, only to be used if you know what you are doing). Also other likely culprits are that you slipped out when doing the shot, therefore you have a little bit of localized bruising around the injection site from leaked blood that's pooled into the tissue surrounding the puncture mark. Also sometimes if you use a 31G or a 30G in those spots and inject too quickly into the vein, you can do a semi (or full, ouch!) vein blowout and that'll DEFINITELY leave you bruised up as FUCK.
So, to sum it up - I'm 99% sure that it's just a bruise. Hand veins tend to bruise incredible easily unless you really know what you are doing, and it sounds like you are new to this. I would totally totally totally totally not going in your hands unless it's being done by someone who has a lot of experience with this, or you have no other place to go. From the sounds of things, you seem relatively new to the whole injecting this, so it might be a good thing for you to do to quit. If you're able to, go for it! I've been off heroin for almost 3 years now thanks to methadone maintenance and it has been a godsend. If you're able to quit dope so easily (I know I wasn't, for like, 7 years!

(( ), kudos to you OP.
One last question(s): a) What rig did you use? b) What kind of heroin was it? Regular east coast powder, mexican powder, afghani base with citric treatment, black tar heroin (#4)? c) Also, did you miss any of your shot at all? d) Did you use a new rig? A dulled rig will definitely help bruise the fuck out of your arms, ESPECIALLY WITH YOUR HANDS!
My guess, is, you used either too big of a gauge syringe for your hands, or a dull one, or you had it slip out or just had trouble registering in your hands.
Download some vein map images on google image search. It'll help you a ton as you really shouldn't have to resort to your hands until your arms and other upper arm spots are completely shot out.