Could be either. Too much nicotine is often accompanied by rapid or irregular heartbeat and/or headache.
Too little nicotine or vaping juices that don't calm you can also cause jitters. When I started vaping I discovered I could not vape food flavors - they left me craving an analog as if I had just eaten. I had to vape tobacco juices with good TH and food aroma. Around the end of the second month I started adding cocoa, chocolate, vanilla or caramel juices to my tobacco juices and that seduced me over to vaping non-tobacco flavors alone. Especially the cocoa since most US cigs have cocoa in them. Other people may find vaping tobacco juices gives them analog cravings and they have to only vape other flavors. And some can vape anything right away. So pay attention to how your various flavors make you feel.
If you recently started vaping, you could just be getting over the 4000 other chemicals in analogs which include things like MAO inhibitors. Remember to drink plenty of fluids to avoid dehydration and take vitamins.
If you are not getting enough TH to satisfy you then use juices with more PG to let the TH through, or with more nicotine, or buy a better ecig. Around 3-4 weeks many people end up buying a different ecig to try. KR808D-1 (like Vapor4Life and SmokelessImage Volt) are real 3.7v slim minis and can give a hotter drier hit with some cartos. I use Vapor4Life WOW cartos. To get a bigger hit than that you have to skip slim ecigs and use fat battery models like Ego or Riva 510 or mods, either with the hotter LR low resistance atomizers or cartomizers.