As stated in your question, you have a couple almost bald patches on your head. This could be the Alopecia Areata, a condition of undetermined cause characterized by non-scarring round patches of baldness. It is said that the disease is inherited ( runs in families) and follows autosomal dominant inheritance. Also, there could be some kind of auto immune disorder behind it. Once affected by it, the hairs regrow within a year's period of time. So do not worry about it. You will grow them back in a year.

Other conditions of hair loss:
1. Alopecia Totalis - It results in complete loss of hair on the head.
2. Alopecia Universalis - Results in complete loss of hair on the scalp and the entire body.

Treatment - No affective treatment has been found yet. The hair regrow on its own with a period of one year in most cases. Doctor may give some steroids to stimulate the hair growth.

sources: Stedman's Medical Dictationary.
http://www.webmd.com/skin-problems-and-treatments/hair-loss/tc/hair-loss-symptoms
http://www.hairlosslibrary.com/hair_diseases.html

image source:http://t2.gstatic.com/images?q=tbn:ANd9GcTK0HVd3nFvCIR6JMf-93Meu9fp4Vm1kef6msJLY8ZGnGzoSilLug
http://t2.gstatic.com/images?q=tbn:ANd9GcRVzDZApEiJUBU7-cv5gkp1An1hxOKiRda5j53wWWsKncR5MVWkAA

I have had this happen a few times in recent years.

Here are a few triggers that caused mine:

1. Birth Control: Hair loss is a common side effect of birth control: http://blog.americanhairloss.org/hair-loss/birth-control-and-hair-loss/

2. Malnutrition: make sure you're getting a healthy amount of food, the hair, skin, and teeth can be severely effected by malnutrition.

3. Lupus is a disease that can cause hair loss, are there any other symptoms? You may also have an autoimmune disease that attacks your hair follicles. You could have your doctor test your blood to see if there are any irregularities.

4. Vitamin deficiency: taking a multivitamin daily will help this. I used to take biotin and folic acid daily in addition to a multi and saw improvements quickly. Some people use prenatal vitamins to maintain the health of their nails, skin and hair.

This website might help: http://www.womentowomen.com/womenshealth/femalehairloss.aspx

There are different reasons for hair loss for different people. There are many factors which influence it. Read the details in the following link n try to know under which category you come
http://laserhairremoval-treatment.blogspot.com/2010/12/causes-or-reasons-for-hair-loss.html

Hair falling out in clumps is not a good sign. Maybe you should talk to your primary care doctor. Are you on a strange diet? Have you been exposed to high level radiation? Do you put harsh chemicals on your head. Be safe, start with your doc, not idiots like me on the internet.