You cannot include medical expenses you paid in 2015 on your 2016 tax return. You can only enter medical expenses you paid in 2016 on your 2016 tax return. If you incurred the expense in 2015, but paid it in 2016, it can be entered on your 2016 return.
So, if I found several medical expenses I paid in 2015 that I did not include on my 2015 taxes, can I somehow report them now...being a year later...like an amendment or something to get credit for the medical expenses I did spend but forgot to include?
You could amend your 2015 return to include them. But be aware that the medical expense deduction is very hard to get. If you were under 65 in 2015, the only part of your medical expenses that even "count" are the expenses you PAID that were over 10% of your adjusted gross income. And then you also need to have enough other itemized deductions to exceed your standard deduction.