Could I be experiencing implantation bleeding and got a false negative on pregnancy test?

Based on tests, you are HIV negative.. Anal pricking is low risk.. Keep practicing safer sex in the future. 

Normally, implantation bleeding is lighter and shorter compared to the usual menstruation flow. A negative HCG test could mean that it’s not a case of pregnancy. The tests may have been carried out too soon for the existence of any pregnancy to be detected. 25 days may also be too soon for an ultrasound. So it's either way too early for these tests or the likelihood is that you are not pregnant. Keep an eye on the symptoms and repeat the test after a week or two for better results.

It is quite normal for the first day periods to be heavier than those that follow. The explanation for this is that the endometrial lining of the uterus completely sheds on day one resulting in heavy menstrual flow. With every day the amount of shedding reduces reducing to a lighter flow. Nevertheless, should you develop unusual heavy bleeding or uncommonly long periods of menstrual flows visited a gynecologist for the assessment and treatment.

A faint line on the pregnancy test can indicate an early pregnancy. The black of a line you are managing could be implantation bleeding when a fertilized egg sticks to the uterus. It is a common thing in the first three months of pregnancy and it's often nothing to worry about. Watch out for it and be gentle with yourself.

Does ipill delay periods? Yes if taken properly as prescribed periods delay due to ipill. The effectiveness of the i-pill decreases the longer you wait to take it and taking it within 48-72 hours is the most effective time frame.If your concerned do a pregnancy test around 2-3 weeks after the date of the last unprotected intercourse. This will give the most accurate result.

It is not usual to have bleeding after 2 weeks of having your period. This could be related to the hormonal imbalance, fibroids, or infections. It is advised to see a gynecologist for a more detailed examination and treatment.
 

If you think the symptoms are of pregnancy then you can take a home pregnancy test to confirm, or do a urine pregnancy test.

Havin two periods in a month is not uncommon. The cause could be stress or hormonal imbalance. It's generally not dangerous, but consult a DOCTOR if concerned. 

You should be good since the morning-after pill blocks an egg from ovulating for a while. Unprotected sex should be avoided in the future.