Dark spot under big toenail

More likely to be a bruise, especially if the type of melanoma you had as your primary was not the kind that shows up under nails. Rare to have 2 primaries of different types. Bruises can take up to 6 months to grow out from under a nail. Show to your doc next time you see them or send them an email with a pic and see if they feel like they should examine in person if you don't have an appt scheduled in the near future.

Is it staying in one spot or growing out with the nail?  Toenails grow slowly so it might be hard to monitor, but often times melanoma under nails is typically a black streak.  Growing out, not suspicious.  Staying in one spot, suspicious.  Definitely take photographs to monitor where it is located. 

Yes, you can have another melanoma, however it is not very common to have a different type of melanoma.  If you are stage IV from cutaneous (skin) melanoma, it's pretty rare to have acral melanoma (under nails, palms, soles).  Typically that melanoma is found on darker skinned persons.  So unlikely but never say never.  If this were a new primary, it's unlikely a PET scan will show it.  Tumors have to be about 5mm in size to show up on a PET.  Sometimes you might get a hit with something smaller but that's typically why new primaries are not caught on scans.

I also have a big spot under my big toenail.  Noticed it last winter during a pedicue.  She said it was a bruise.  So in May with everything going on with melanona i went to my dermatogist.  He took a picture of it and i go back the end of this month.  He didn't seem to think ???? it was anything but wanted to watch it and possibly refer me to podiatry.  It has not moved in over 6 months.  To me it did look like a dark mole, again dermatologist didn't think so.  Don't assume, go see a dr.