What Causes Thickening Of Endometrial Lining Of Uterus?

What could cause my endometrial lining to build up to 28 mm (2.8cm), possibly more now? HPT has been showing up negative. I am only 24 years old, and the doctor told me that this is not normal. I did have unprotected sex during my most fertile days last month, but still waiting on the blood results. This has never happened before, and just around April 7th, I had went to the doctor before after having spotting 4 days after an abnormally light period from april 23rd-30th, they did an ultrasound and I was told my endometrial lining was around 9 mm, which was far too thick to have just had a period apparently. Now it is up to 28 mm, so this isn't something that has been building up over time, it has just happened this month. I suspect pregnancy but the HCG just isn't high enough or perhaps a chemical pregnancy took place, but I can't imagine any other reason why my lining could still continue to grow like this. I had a check up in february and everything was normal, lining size and all. Now I have been having this pain on my lower right side for the past 4 weeks, sometimes its a sharp pain, sometimes it's a pulsating dull pain, and at times it spreads to my lower right back side. What's going on? Could an ectopic pregnancy be of concern to me?