Had Normal Period. Pain In Lower Back, Cramping In Stomach. Normal With New Birth Control?

Hi , im 23 years old and have been on birth control for almost two full weeks. i had a normal period for the first 5-6 days, spotting for about 4 then i stopped bleeding completely, then today, i suddenly have pain in my lower left back, and cramping a little in my lower left tummy. but not like period cramping, like a dull continuous poking pain, and brownish spotting which turned into dark red bleeding. This may be normal, but im new to birth control and