It could be an insect bite, an inverted hair follicle or even a pimple. Does it hurt or itch? Is it an odd shape? If it continue to worry you, see a dermatologist, they will be able to set your mind at ease promptly, however, from you description, I would say it is nothing to worry about.
You should be able to insert a picture in your profile under pictures.
It is difficult to figure out what this bump could be by just the description you have given. When you say for a while, is it days, weeks, months? Do you recall being stung or bitten? What part of the leg is the bump on? Does the bump hurt; does it itch; has it got any bigger since you noticed it; does it have a head like in an acne spot for example; do you recall falling on the grass?
Have a closer look at the bump to see if you notice any puncture marks. Spider bites would leave 2 very tiny puncture marks.
Apply antiseptic cream to the lump to see if that will help.
If you do notice it starts to enlarge, pulsate or cause any discomfort, make an appointment to see your doctor. Sometimes a pharmacist can give you advice, so it is well worth asking if the pharmacist can take a look and recommend any particular cream for the pump.