When will my iron pills start working?

Hi, I am a 13 year old girl. I recently had my blood drawn at a doctor's appointment, and I found out that my iron levels are pretty low. It made sense, because I have been experiencing a lot of the symptoms of an iron deficiency- I've been super tired lately (bags under my eyes too), sometimes get lightheaded and dizzy, have been getting bad headaches, have somewhat pale skin, grooved fingernails, sometimes feel short of breath, and I grow really slowly, I've always been underweight (I'm 5 foot, 84 pounds). I was prescribed for some iron pills, which I just started taking two days ago. I mean, I didn't expect them to magically cure everything as soon as I took them, but when are they going to start working? I had a headache today, and while I was standing in my room I suddenly felt dizzy enough that I lost my balance and started to lean over to one side! That has never actually happened before... I also have an anxiety disorder, so when I get these symptoms it can sometimes really stress me out. I am a really active person- play soccer, basketball, run track- but lately, I've either gotten a bad enough headache, felt so lightheaded, or just been tired enough that I haven't been able to play in my soccer games or run my track meets! I feel like I'm letting my team down, and I just want to get back to the way I used to be. My question is, when are these iron pills going to kick in, because I'm tired of feeling bad all the time! Also, any tips on coping with headaches and lightheadedness?