Could I be showing early pregnancy?

I presume you mean your period bleed started on 8th Dec and lasted 5 days. Your cycle sounds like 28 days.

If your period is late, the chances are you ovulated late, although worry is thought to have an impact. To become pregnant from 25th Dec sex, you would have needed to ovulate from 25th Dec onwards. If you ovulated 25th Dec, then you would not expect a period to start until 8th Dec, so a test before then is likely to be negative anyway.

Hope this helps

The ovulation calendar is not precise, but works on assumed normal variations. So you would be expected to ovulate on 20th, and a variation of two days either way gives you the 20-24. But you actually only ovulate on one day! But you may have ovulated a week late. On the other hand you may not have ovulated at all.