I got diagnosed with a 5mm pituitary adenoma (non functioning) 2 years ago, now they say it can't be seen on an mri yet I still have my symptoms, like migraines often, constant headache and sickness. Help?!

Likely not related. Your migraines are not related to the pituitary lesion, which may have merely represented a non-functioning adenoma. If you have constant headaches, would advise starting a preventative agent, and both prescription and OTC preparations could be considered.
May not be related. a 5mm adenoma may not be the cause of your migraine. Please consult your PCP for an examination and appropriate migraine medication.