Help!!! Vanishing positive :’(

please help!! My partner and I are both female and have been trying to concieve for the last 4 years since our son was born sleeping. My partner is 4 days late for her period so we tested today on a superdrug test left it alone in the bathroom for 5 mins and when we went back there it was, a faint but pink and thick second line, we were over the moon and the line lasted for about 10 minutes then slowly disappeared until now it’s hardly, hardly there. I know that we need to test Again in a couple of days but has anyone else been in this situation and if so what was your end result? thankyou for reading xxx I have attached a photo of what the test was like when we went back