Hi, how fast did these come on? I got something similar when I was pregnant, and the dermatologist just shrugged it off. But I didn't get it on my hand. Yours might be more like a generalized rash than mine was. (It almost sounds like the measles. Have you been vaccinated against them?)
Pantothenic acid is a B-vitamin, right? What are you taking it for? (People apparently take it to do a lot of things but in some instances there isn't a lot of proof that it does that thing.) It would be easy enough to tell if the pantothenic acid is giving you this rash. First, check that you aren't taking an excessive amount compared to the recommended dose, and if you are, adjust it downward. And second, you could see if there is a link by simply stopping taking it.
If you do stop the pills and the rash continues, I'd go see the doctor.
Well, certainly if you are taking fifteen times the recommended dose of a supplement, and then you get funny spots, the first thing to do is to stop taking the supplement, and see what happens. I've got to leave now but when I get back on, I'll look up Vitamin B-5 (or whichever one of the B vitamins it is) and see if that is a known side effect. The other problem could be that the particular brand you bought has impurities in it, meaning you might not even know what is in the capsule.
Don't scare yourself, it doesn't sound like syphilis to me.
The reading I did says there can be side effects from too high of doses, including calcification of the blood vessels and other side effects involving hormones, and they (hormones) are also sometimes part of the cause when someone gets hyperpigmentation. If you google "side effects of pantothenic acid" you can see the recommended max dosages in grams, then read the bottle and see how much you have exceeded the dose by. You could go to the doctor and tell what you have been doing and see if he or she has an idea of what might have caused the spots.
The simplest thing is to stop taking any of it, and to flush out your system with juice or water for a week or two and see if it helps. If the issue is caused in a straightforward way by the supplement, it seems like it should clear your body pretty fast and the spots should go away. If it's hyperpigmentation caused more indirectly by the effect of the pills on your hormones, well, hyperpigmentation takes a while to go away (be sure to sunblock the patches).
I would also suggest that if you do get to the doctor, get a scrip for an antibiotic to clear up your face in a couple of weeks, so you can stop doofing around with folk remedies -- ('this girl on YouTube' is not a doctor looking at your particular case with medical training). If you are still anxious about STDs, you could also get tested for them. And of course it goes without saying that if you give up on the Vitamin B5 and you still have the spots, a doctor is your best friend to determine what they have come from.
You probably don’t have syphillis but an allergic reaction to what you been taking. Do you eat a lot of acidic foods such as oranges, tomatoes, etc? If so you might want to eat those in moderation since you are developing a rash. As others suggested, do go to a doctor or dermatologist. Also, if you are sexually active it won’t hurt for you to get tested but your worries shouldn’t be syphillis.