If it's an much higher nic than the 12mg you're used to,
1) I don't think the nic absorption would be that fast that you would feel the effects after only 5 to 10 vapes. Someone correct me if I'm wrong.
2) you would definitely feel a huge difference in throat hit if the nic is much higher than 12mg.
I would look at other causes instead of zooming in on the nic levels. But I'll still keep an open mind as each person's tolerance towards nic might be different.
You said correct you if you are wrong, so I will
A person with can actually have a nicotine overdose (not lethal, but where side effects are present, such as the side effects the OP mentioned) from as little as ONE to TWO puffs. From either a cig, OR an e-cig.
I've been using 18mg/mL since I started vaping, back in May 2013. I just started dripping using an RDA about a month ago. I'm using a dual coil RDA at about .5ohms. If I were to use 18mg/mL, like I was using in carto tanks, or clearos, and took a drag for 5-10 seconds, I would get unpleasant symptoms from too much nicotine. I don't even want to IMAGINE what would happen (side effect wise) if I did that for 5-10 puffs.
As for the OP, since we weren't there, the ONLY thing we have to go on is your original post. Therefore, I'm going to assume NOTHING was changed regarding your diet, meds, daily routine, or vape routine, except for using a new e-liquid supplier. If nothing in 3 years has recently changed, then I would assume the supplier used a diff. nicotine amount then 12mg/ml on accident.