3 Miscarriages, Referral to Infertility Clinic?

I'm almost 28, I have had 3 Miscarriages all around 5 1/2 Weeks, within 9 Months. I already have a Son so still very confused as to what's changed and why....?? I had an Appointment with my Dr again today and got my Referral to the Infertility Clinic, I was just wondering what to expect at the appointment? What kind of test will get done? I Hate not knowing why this has happened and so confused, I don;t understand why I can become Pregnant fast enough but can't carry for more than a couple weeks. It's always on my mind and I just need to know why now so I really hope that this Clinic can help! Thanks for any advice on what to ask or what will happen, much appreciated.