bbt thermometer

I have I reli-on thermometer that I bought at Walmart thats strictly for ovulation and I've been taking my Bbt every morning at the same time. Today I took it at 7:25 and then at 7:35 and then at 7:40 and I got different readings sometimes they were lower than the one before but then sometimes they were higher than the one before. For instance at 7:25 I got 97.42 then at 7:35 I got 97.47 then at 7:40 I got 97.42. Then I got up to use the rr and after I took it just to see and it said 97.37. So is mine just not working right?!