ttc but not sure about when

my HS and I made love the 28th of last month. I'm got 3 negative pregnancy tests at 1wk so I was going to put it out of my mind. I'm very regular so I don't want to blame symptoms on my period. could I still be pregnant? I know twins won't be determined until an ultrasound. this would be my 5th pregnancy I have a little boy who is 4 and very healthy. I have textbook symptoms that seem to come and go, however it's so early on this would be 2wk..I really can't determine anything. any advice?