Hey sweetie,
It's a perfectly normal thing to worry about... I have the same issues as yourself
I used to be a size 8-10 before having my little boy but am now a size 12/14 and although my stretch marks aren't very noticeable I do have what I call my mummy-tummy. I work for an agency, and although I did feel very self-conscious at first, I can honestly say I haven't had any complaints.... in fact nothing but compliments (actually, the clients are more obsessed with another noticeable side effect of having had children, the boobs, and don't seem to notice the tummy that goes with it!). I completely empathise with you though as for me personally, it's knowing what you used to be like before and knowing that you've changed that makes you feel like shit! However, the more clients I see the more I believe that just because I am slightly bigger, and a lot curvier, doesn't make me any less attractive than I was before. Sometimes this job can actually be a confidence booster. You'll see. Us women come in all shapes and sizes, there is a market out there for all of us... yes some men like slim women but just as many prefer a curvier shape. I really wouldn't worry. Besides, agencies take photos, so if a guy has booked you then he has more than likely seen your photos and you are therefore exactly what he is looking for!
Remember that nobody is perfect..... especially the punters themselves, and we all have flaws, be it stretch marks, scars but it makes us who we are
One hint of advice though.... it might be a good idea to take a basque/corset or negligee to your photoshoot, something which flatters your shape. That way not only will you feel more confident but your photos will be better ( I didn't and really wish I had now).
Good luck sweetheart I'm sure you'll be fine!
Love
Sophie
xxx