hav got a lump in my right armpit..

Hi, welcome to the ehealth forum and I am glad to help you.
You seem concerned by lump under your right armpit and you also made a mention of irregular menstural periods.
Any lump under the armpit (axilla) is concerning. It could be something as little as a cyst or inflamed hair follicle, but suspicious lymph nodes also present in this area.There is a long list of possible causes. In many cases, these things will resolve on their own over a week or so. If it becomes increasingly sore or painful,you may need an antibiotic for which you should see your doctor.
There might be no connection between lump under your armpit and irregular menstural periods. There are number of factors such as stress, dieteray problems(anorexia, bulemia or simply poor diet), drugs etc. which can affect your menstrual period.
You must consult your doctor for definitive diagnosis as the same is not possible online.
Hope this helps. Take care.
Note: This post is not to emphasise final diagnosis as the same cannot be made online and is aimed just to provide medical information and no treatment suggested above be taken without face to face consultation with health care professional.