Thanks for the query.
Bleeding in early pregnancy
can occur due to many causes.
But as your pregnancy test was negative, there is less possibility for pregnancy.
Dark color of the blood can indicate collected blood.
So,possibly that could be your regular period only.
If you want to rule out, once go for blood test
for pregnancy. It can give accurate result compared with urine test.
In the next cycle, you track your ovulation and plan intercourse around that period.
For more details : http://srsree.blogspot.in/2013/10/ovulation.html
This will increase the possibility of pregnancy.
Start taking folic acid
supplementation from nowonwards.