When is it best to take a digital pregnancy test?

Hey ladies I have now tested positive 4 times using strip test but when I tried a digital today I got a negative. I am now 3 days late on my period and very confused. I am having a lot of symptoms. Sore heavy breasts, headaches, cramps, nausea, tired, peeing all the time, heavy abdominal area and I'm always hungry. Please help.