Thermometer Issues

Ahh!! My bbt is all wacky. I have two thermometers though, one I bought through Amazon, the other from Walmart. At first, I was using the Amazon one and I really liked it. Then I noticed for 3 days in a row, it was giving me the same temps. I thought, hmm let me see if it will give me that temp at some other point in the day and it did... TWICE! I knew something wasn't right so I bought a new one. The new thermometer from Walmart seemed to have been working fine but my temps are very "Rocky Mountain". I said let me see my Amazon one, which is no longer giving me that one temp and it seems like my Amazon one gives a higher temperature. I only record what my Walmart one gives me to be consistent, but I think the Amazon one may be more accurate. What should I do?