Dollar store tests

Ok I gotta get your guys insights. I'm 21 Dpo. Last month I took a test and it came up positive then negative and period came next day. This time, it was positive, and then it disappeared again! I only buy them because I'm 7 months ttc and I got tired of buying more expensive until I am more late.. Like this month. Have u guys dealt with this from those cheap tests? Also! It was all between the 3-6 min mark of the tests! Top pic is from when it disappeared