reason for concern?

Hey June mommies to be!  Looking for some advice/reassurance.  I'm 16 weeks 4 days according to my dating scan, although by my own calculations I should only be 15 weeks 5 days, it's my first pregnancy.  I'm not due for my 20 week scan for another month or so and I'm starting to worry.  I've not gained any weight and none of my clothes are tighter.  I know they say you don't always 'show' so early but surely my tummy should feel different when I touch it? I had a wee podgy belly before I fell pregnant but it hasn't changed at all.  Obviously all symptoms of early pregnancy have now passed which makes me more paranoid.  The only thing that's reminding me I'm pregnant is a constant stretch and strain feeling all across my front that I've had since 7-8 weeks, it is slowly getting worse as the days go on, I can't turn in bed, I can't walk until a few seconds after I get up off a chair etc, I can't left my legs to put trousers on.  Do you think any of this is a reason for concern, I have a midwife appt on Thursday but they don't generally do heartbeat checks or ultrasounds unless you have bleeding or cramping which I've had neither of.  I see lots of photos of bumps on here from people at the same stage and I'm just worried that nothing at all is different for me.  I generally get stretch marks too at the slightest weight gain and so far nothing! 😔 can anyone reassure me at all?