BBT how to measure?

Catia

hello! i've started this month to use glow and register daily all Info including bbt.

at the beggining i used to measure the temp with digital thermometre and placing ir under the arm. I have red about Better to place it in the mouth and now i have a Big diference um temp registry só i'm confused. i was marking 36,3' and now 36,8'. is this normal? i'm on day 1po and my cycle varies From 25 to 28 days