What is the pebble under my skin where stitches were?

I would definitely follow-up with your surgeon to make sure everything is ok, but it sounds like it could be a milia.  These occur along the suture line after surgery and are small inclusion cysts that form.  They can usually be treated in the office with a small procedure to excise them.  Either way, your surgeon should be able to take a look and figure out what is going on.

Occasionally epithelial inclusion cysts occur. The risk of suture granuloma formation is decreased by the use of certain sutures which must be removed completely at the appropriate time. The most common reason for small lumps occurring along the eyelid incision is due to lymphatic fluid retention or a small amount of scar tissue. In the period immediately following the operation, the scar tissue is usually raised and feels firm. Over, the next 3 months or so, the scar will mature and soften. Usually, intervention with this is not required; these scars respond well to routine scar treatment like moisturising creams and silicone gel. Occasionally, a small dose of cortisone injected into the scar which would help break up these small, hard lumps. A lump or granuloma sometimes forms as a reaction to suture material used in eyelid surgery.You can consult with your doctor about it if it causes discomfort.