If you had miscarriage on 15th December, then if you had sex on 17&18th, it is unlikely that you would get pregnant as ovulation would undertake once the hormone levels recede back to normal and you have your normal periods again.
If you have started experiencing cramping and bleeding again then it is likely that your menses have restarted and is common to see after therapeutic abortions to have frequent polymenorrhagic cycles. It is suggested that you may undergo an ultrasound transvaginally to look for endometrial thickness and rule out ant remnants of products of conception. If all is well, then you may just follow barrier contraception for at least 3 months before you can plan for another pregnancy to allow the uterus to in-volute naturally to its pre pregnant size.
I hope i have answered your query in detail,
wishing you good health,
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