Hello, and sorry to hear about your bruise, I can understand how this would seem odd if you didn't remember doing anything.
Based on your description, my best bet is that it's simply a broken blood vessel, that you did incidentally, possibly while you were carrying groceries. These can happen when you hit something just wrong or strain something, and the blood leaks out and you've got a bruise. These are actually fairly common in the finger, and they'll cause the aching like you describe. The good news is that they'll go away on their own, and they aren't a sign of anything more serious.
So I'd give a couple more days, take some ibuprofen, or through some ice on it if you think it would help, and you should be good. As far as the dizziness, this is fairly common too while staring at a computer screen, so make sure you take some breaks while your working on the computer, and don't worry about that either.
I wish you the best of luck, and please reply if you have further questions. Best wishes...
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