Early pregnancy bleeding, week 5-6, low BHCG 300miu/ dl, took IM progesterone for two days, bleeding decreased but not stopped, is this abortion?

No. Not necessarily. Should get bHCG 48 hours after the first. That will tell you if its doubling normally or dropping.
Early preg bleeding. Most bleeding in the early part of a pregnancy is not due to miscarriage. If we are suspicious, we generally check the hCG levels more than once generally 48-72 hours apart. The rise should be about 65% or more. Once the levels get high enough your physician may consider an ultrasound exam.