Advice needed- possible signs of miscarriage?

Hi all,

would greatly appreciate some advice as I'm climbing the walls, This is my first pregnancy.

I had a positive preg Test last December (19th) and went for my dating scan last Wed to be told I wasn't measuring right with the dates I'd given and was only measuring 6 weeks and no fetal heartbeat detected.The midwife and doc asked me is it possible I got my dates wrong? Also between late Nov and December last year I changed from a progesterone only pill to a combined pill, my pregnancy wasnt planned and presumed I fell pregnant in that change of pills.

the midwife thinks that the pill changes caused by body and cycles to go crazy and possibly gave a false positive test result.

my periods were irregular.

is it possible for changing pill types to cause this? I just can't understand why I would test positive for pregnancy in Dec if she thinks I conceived in January? I did 3 pregnancy tests in Dec.

I have had bleeding some fresh, some old and not really any cramping. bleeds arent really that heavy.

I have another scan next weds to confirm if there is a heartbeat. I just don't want to get my hopes up, i had preg symptoms in Jan and they stopped. i don't feel pregnant and think it might of stopped growing at 6 weeks but doc and midwife are trying to get me to remain positive. They said the sac and baby measure correctly for 6 weeks as per last weeks scan. Im so confused and would like some advice... 

xx