It is normal for someone who has had a tummy tuck to start bleeding at the site of the cut every once in a while?

Depends. Within the first few weeks after tummy tuck surgery, some drainage is not uncommon. However, bleeding is not expected after the initial postoperative period.
Possibly. In the early postoperative period it is not uncommon for the incision site to bleed when irritated.
No. If it is early after the surgery, it may be because of a wound gap or skin separation. If it is after a few weeks, it means that the scar is irritated or inflammatory, either because an infection, suture rejection or hypertrophy rubbing on clothes, or scab separation. You will need to show it to your surgeon.
Yes and No., Early on a little surgical site bleeding is not out of the ordinary, but anything more than just a spot should be reported to your surgeon. After a week or so assuming the wounds are closed, bleeding is unusual and should be similarly brought to the attention of your surgeon.
Depends when op done. Within the first week or so after surgery when incisions are not yet durable, excessive activity or stretching could open up a minute portion of your incision. If you are further in your recovery, there can still be areas of breakdown such as where dissolving sutures can break the incision surface. Normal is relative; ask your surgeon who knows the details to check out your progress before problem.