Had sexual intercourse at night at 00:00 then woke up at 4 am from sharp…

Had sexual intercourse at night at 00:00 then woke up at 4 am from sharp stabbing pain and it pulled in my back strongly Then pain. I calmed down but it still pulled my back strongly and my stomach tense as if Second birth of the first daughter, 16 now 35 weeks and 5 days according to the screening, the first one is about 2500 peletina Is this training? Or has it started? Is it scary to give birth at 36 weeks?