Cramping on and off for nearly 3 weeks

On 5th of may I had period cramps like I came on went to toilet wiped a little bit of dark blood thought my period came on early but nothing then on 7th and 8th wiped blood once thought my period come on ever since then nothing on the 12th may I did a 10miu sensitive pregnancy test it was negative 3 days later 1 did another one fmu and it was faint did another 2 days later it was faint then brought another brand 10miu pregnancy test fmu on 21 may had a faint positive and I'm cramping on and off no period tested with a 25miu first response today and negative went to doctor said he won't do blood test wants a urine sample told him I used sensitive test