Ovulation tests getting lighter πŸ˜–πŸ˜” any help????

Amy
Hi everyone,
We are trying to conceive baby no.2 but my cycles can be so irregular due to my underactive thyroid. Anyway I thought I'd get some ovulation tests to get a better idea of when in my cycle I'm actually ovulating. 
So I took one yesterday and the line was dark but I thought it was negative as it wasn't quite as dark as the control line. Took another today expecting it to be darker but it's lighter than the one yesterday!!??
 Like I said it's the first time I've ever used these so I'm not fully experienced in how they work, I was just wondering if anyone could shed some light on why it was lighter today, and if anyone has any general advice for using them or if anyone has had any success with falling pregnant while using them.
Thanks everyone 😊😊😊 xxx