On last day of last period, had unprotected sex no ejac. One week later I have dark brown spotting for the past 2 days. Is this implantation bleeding?

It may be so. It is possible, so weight till your periods are due and if you miss your period, that means you are pregnant. If you want to know earlier you can go to your doctor and get blood test for pregnancy which becomes positive 3-4 days after implantation, and early pregnacy urine test 5-7 days after implantation.
Take a test . If you think you might be pregnant check then check a home pregnancy test. They are cheap, reliable and available at any pharmacy.

Related Questions

Spotted immediately after unprotected sex. Have had brown spotting for two days. Period was supposed to end 2 days ago. Taking Yaz (drospirenone and ethinyl estradiol) birth control. Help?

Breakthrough bleed. Breakthrough bleeding (bleeding at the wrong time) is very common on birth control. It happens in about 1/3 of users. Its no big deal and usually self corrects over time. If you are not satisfied or it continues to happen then consider using a different birth control method. It is a common side effect and does not mean something is wrong or that the pill is not working. Read more...
Post coital bleed. If you had some remaining menstrual tissue that hadn't been expelled during your period, sex may cause it to come out. Female orgasm and sex in general causes some uterine cramping (contractions). The other concern would be a cervical infection and bleeding from the penis hitting the cervix with subsequent bleeding. Read more...