Welcome to OCF!
What type of cancer did you have? Bloodwork will not necessarily show you have oral cancer, at least that is correct for squamous cell carcinoma (SCC). You may have some problems with your thyroid or anemia making you feel run down.
The first time I had cancer I felt lousy. I felt like I was just beginning to get the flu, achy and just not quite myself. Second time I noticed the lesion but felt ok, the third time I was diagnosed I thought for sure it must be a mistake. I felt healthy. The day before my doc told me I was Stage IV, I went on a 25 mile bike ride with my son. I had never felt better but I was really very ill. So you see, you can never really be certain at least that was my experiences.
You should be having follow up check ups by a qualified ENT. What about scans? I would suggest seeking out a second opinion at a large cancer center where they have a better chance of being familiar with rare cancers.
Cancer centers