Blood clots/DVT's can be really painful but sometimes they can be present without much pain too. If it were a DVT (deep vein thrombosis) then generally the swelling is higher than just the ankles and it's very painful to stretch the calf, and often feels quite hot. DVT's (clots in deep veins), either lower leg or thigh can be serious and travel to the lungs. Clots in superficial veins (closer to surface of leg) are generally harmless. If you have access to physician care, it can't hurt to ask for a doppler ultrasound to rule out DVT. If you haven't done so already, I would strongly suggest getting compression stockings at 20-30 mm Hg compression. They can help reduce the risk of blood clots (hence why they're recommended for frequent airplane travellers). Sometimes there are over the counter compression socks (18-20 mm Hg compression) which can be effective at reducing swelling also (but less so), and they're usually cheaper and readily available in places like Wal Mart. Even just knee highs can make a difference (the majority of compression happens at the ankle. Swelling can be a sign of lymph or vein vessel problems. Pregnancy hormones make vessels more prone to stretching which makes fluid flow back up the leg less efficiently...a major reason many blood clots develop - blood pooling, slowing down, getting sticky and clotting.
The compression stockings, when properly fitted, help the lymphatic vessels and veins work more efficiently, prevent permanent damage to the vessels...and your legs will thank you for them for years to come! Every pregnant woman should consider them. Aching and swelling are early signs of varicose vein development - nipping this problem in the bud can save many many future hassles!