Some guys have very watery semen, others have it very thick, and some have the type you describe, which is very stringy and/or lumpy. Likewise, there can be variation in coloring, odor, and taste. I suspect it has to do with a lot of factors including how often or how recently you've ejaculated and perhaps your diet or fluid intake.
Could also have to do with the fact that masturbation may be a quicker process than when you make love with your girlfriend and the excitement factor is differant.
Did some rsearch and someone has come up with a probability:
The source and answer to your concern probably lies within one or more of the three seminal fluids. When you ejaculate what comes out is a mixture of three fluids – called semen. Seminal vesicles fluid, prostate gland fluid and the bulbourethral glands fluid. The seminal vesicles fluid and the bulbourethral glands have a mucous consistency and the prostate secretion gives the semen a milky appearance. The liquid semen coagulates (thickens) rapidly due to a special clotting enzyme produced by the prostate gland – that then reacts with a substance produced by the seminal vesicles. Depending on the chemistry that day of your bodily fluids – the semen may be clotting faster than usual creating the globules. Yellow(ish) and or white(ish) or even clear ejaculation is normal.
It is normal for the appearance and texture of young men's fluid to change from time to time.
