Getting tanned = UV rays penetrating your skin, causing oxidative stress, melanin leakage, etc.No sunscreen protects against 100% of rays.
Saila, I have been pondering this, and I really think that what someone said earlier was correct: that you did not apply enough initially. Also, make sure your sunscreen has appropriate UVA ratings. Those are the rays which are thought to cause the release of melanin which combines with oxygen to oxidize and color the skin. I have found that a lot of sunscreens from the US (I cannot remember where you are located) do not have enough, even though they are rated as "broad-spectrum". I switched to a sunscreen rated PA+++ (I think this is from the Japanese system) and have had significantly less of this problem. As far as re-applying with makeup, I make sure I have a sunscreen with plenty of zinc oxide which is said to block most of the UVA spectrum.