My periods are getting delayed after having sex with my partner. But when i tested my pregnancy in the morning using pregnancy kit it turns into negative. Am i really pregnant? If not why my periods are getting delayed?

Hi, I am a girl with normal 28-30days menustral cycle. Bt in this month my cycle is missed for 2days. My last period started on 3rd Dec. As per my calculations based on previous months, it should be started on Jan 1st. I had sex with my partner 2 times on Dec 23rd and Jan 1st(as per my calculations, it is a safe time). So on Dec 23rd he just come inside my vagina but not finished. But on Jan 1st he ejaculated inside me. I am so much worried that I am pregnant. I tested with pregnancy kit 2 times (once in morning time) and showed negative result. But am still afraid. Is this too early to test? Do I need to do a clinical test? Is it good to have a early period pill? So that i can initiate my period and confirm i am not pregnant. Do i need to a blood test to confirm that or is that too early to detect pregnancy?