Could the doctor be wrong?

hi guys. just need some advice. i went off the pill recently. my last period was 13th may. i took a test a week or so ago and it said i was pregnant. i took another 3 to make sure. i went and had a ultrasound to see if they could see anything but they said they couldnt see a sac but some thing was thickening which apparently was a good sign. however my doctor rang me yesterday and wanted me to come in. i did and she said because there was no sac things are not looking good. she thinks im just having a late period with a false pregnancy test... but remember i took 4 in total.... i asked could it just be that i was to early and she said maybe but she doubts it. this has really upset me as im recently married and this is our first baby. i went for a blood test today and have to go for another friday and go back to doctors monday for results of the hcg levels. but it may sound weird but i kinda feel pregnant. im feeling bloated all the time, tired, constipated, sore nipples....just no morning sickness etc. could the doctor be wrong and i am just early then what i thought (so instead of 5 weeks could only be 3-4 etc)...