anyone?

has anyone ever had their bbt drop below coverline at 18dpo when they were 4 days late for af and still be pregnant? mine dipped but was above the coverline at 17dpo and I had some light brown cm but wouldn't consider it spotting at 18dpo it dropped below coverline and I still have light brown cm (sorry tmi). until now my luteal phase has been between 12-16 days but is usually 14 or 15 days long. I tested the evening of 14 dpo and it was negative and I've asked my husband to pick up some tests so I can check to see if there is any hcg in my system because if it is a chemical then I want to be able to let my doctor know. I am wondering if anyone has had a below the coverline drop and still been pregnant? also it has been a stressful

couple of days (18hr work day due to unexpected work event at 17dpo and another 5 stressful hours on 18dpo) and I have had a sore throat and achy muscles for the last three days. could being sick and the stress cause a drop in bbt? thanks in advance! I'm so confused about this cycle.