The weakness may stem from having heavy periods which could cause Anaemia. As for the absence of periods, I dont think it directly links to Anaemia but could be due to something else. Like you, I am 18 and have suffered with absence of periods. When I saw my Doctor, it was suggested that I might have Polycystic Ovary Syndrome (PCOS). I had a blood test which aren't very good for diagnosing PCOS because they often come back inconclusive. Another way to diagnose this is through a scan. This could be an external or internal scan, although internal is better. However, if you are not sexually active, they probably wont give you an internal one. The best way to conclude if you have it would be through the symptoms you have which is a rather crap way if you ask me. I dont want to scare you, but PCOS is a possibility. Have a look at the symptoms online and if you need anymore advice, message me. Hope this helps.