If it were me I would wait for my period and take the pill the following Sunday. The likely hood that you are pregnant after a negative blood test are slim. There are hormone changes that take place in the body immediately if you are pregnant and a simple blood test from your doctor would reveal that. But not all test are accurate. If you still haven't had your period in a week, return to your doctor and repeat the test along with a pelvic exam. The uterus will be noticeably swollen early on and can be felt in the pelvic exam. You may be able to feel it yourself if you slide your middle finger deep inside your vagina. Always confirm with your doctor though.
Firstly you should confirm the status of pregnancy by getting a serum hcg test which is more specific than urine for pregnancy test. Once pregnancy is ruled out, then you can take oral contraceptives which contains a combination of an estrogen (oestrogen) and a progestin (progestogen). You can start them on the first day of your menstrual periods or first Sunday ( Sunday start) after your periods start.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.