There are a number of things to explore. Phyical causes of being tired all the time include anaemia, thyroid problems and some hormone problems. You would need blood tests to look at haemoglobin and red cell counts, thyroid levels and other hormone levels.
Non-physical problems would include depression, and this would include post-natal depression.
Following childbirth it is possible that you became iron deficient, and this would have been exacerbated when your periods restarted. Your daily intake of iron would not have been sufficient to restore your iron levels. But do not take any iron supplements until after your doctor has run the tests. Getting the dose right is very important.
There are other, rarer anaemias however a Full Blood count would highlight areas to explore.
If you are a stay at home mum, it is possible that you are missing adult stimulation, and combined with the normal post natal blues, this could be a trigger into clinical depression. Once the physical things have been explored then consider making an appointment with a councellor to determine if your tiredness is more psychological, it could also explain the lack of interest in sex as sub-consciously you associate intercourse with pregnancy.
Finally there is a condition called post viral fatigue, a simple cold or viral infection could have left you with a long running tiredness.
As you can see there are a number of possibilities and you will need to see your family doctor to explore the causes. Doctors are very used to this problem so before you go along, make a list of all the things you can think of that are related so you can discuss them in detail. Ideally leave the children with someone else so you can have an uninterrupted session with the doctor.
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