Could be a number of things, some of which are mentioned above.
People react differently, absorb differently and I believe you are using a different juice from the original carts. Some PVs get through more liquid than others too.
How much nic you are getting also depends on how much you are vaping, how deeply inhale and hold the vapor.
If you think it might be too much nicotine, try cutting (diluting) some of the juice with some VG.
I agree completely.
My advice: Visit the DIY e-liquid forum and take a look at some of the cool recipes for flavored liquids.
Then: Get yourself a bottle of food grade glycerin ("VG"), some distilled water, and some Lorann flavoring.
Mix up some of that and then make various strengths of nicotine-containing liquid using your 24 mg stuff. The flavoring will help you forget that you're vaping weaker e-liquid, and if the nicotine is what's causing you to have a headache you'll be able to lower the content but raise the enjoyment.
I love either 0 mg homemade e-liquid (just glycerin/water/flavor) or low-nicotine VG-based liquid, cut with VG/distilled water (about 80:20 ratio glycerin to water).
As an aside: If your nic-liquid isn't coming from the UK it's probably coming from China. I'd take the nicotine mg rating with a grain of salt. My hunch is that it's nowhere near what they say it is in many cases. Just a hunch.