Depending on the term sheet you signed when you first invested in the company, you may have the right to invest in follow-on rounds without diluting your equity. However, if you choose not to invest in subsequent rounds, you may very well find yourself owning a smaller piece of the company than you do now. My advice is to check with your attorney and see what anti-dilution protections you were originally granted. If you decide to try to re-cut your deal with your partners, it's better to have that discussion sooner than later.